The Byzantine historian Procopius (born c. 500 AD) held a high civilian post on the staff of Justinian's great marshal, Belisarius. He participated in several campaigns, including the victorious one against the Ostrogoths in Italy (536-540 AD). In 562 he became Prefect of the City in Constantinople. In the year 537 AD Ostia and Portus figure prominently in The Gothic War (De Bello Gothico), part of Procopius' History of the Wars of Justinian, that was written in Greek.

De Bello Gothico V, XXVI
1. Now Vittigis, in his anger and perplexity, first sent some of his bodyguards to Ravenna with orders to kill all the Roman senators whom he had taken there at the beginning of the war.
2. And some of them, learning of this beforehand, succeeded in making their escape, among them being Vergentinus and Reparatus, the brother of Vigilius, the chief priest of Rome, both of whom betook themselves into Liguria and remained there; but all the rest were destroyed.
3. After this Vittigis, seeing that the enemy were enjoying a large degree of freedom, not only in taking out of the city [Rome] whatever they wished, but also in bringing in provisions both by land and by sea, decided to seize the harbour, which the Romans call "Portus."
4. This harbour is distant from the city one hundred and twenty-six stades; for Rome lacks only so much of being on the sea;
5. and it is situated where the Tiber river has its mouth. Now as the Tiber flows down from Rome, and reaches a point rather near the sea, about fifteen stades from it, the stream divides into two parts and makes there the Sacred Island, as it is called.
6. As the river flows on the island becomes wider, so that the measure of its breadth corresponds to its length, for the two streams have between them a distance of fifteen stades; and the Tiber remains navigable on both sides.
7. Now the portion of the river on the right empties into the harbour, and beyond the mouth the Romans in ancient times built on the shore a city, which is surrounded by an exceedingly strong wall; and it is called, like the harbour, "Portus."
8. But on the left at the point where the other part of the Tiber empties into the sea is situated the city of Ostia, lying beyond the place where the river-bank ends, a place of great consequence in olden times, but now entirely without walls.
9. Moreover, the Romans at the very beginning made a road leading from Portus to Rome, which was smooth and presented no dificulty of any kind.
10. And many barges are always anchored in the harbour ready for service, and no small number of oxen stand in readiness close by.
11. Now when the merchants reach the harbour with their ships, they unload their cargoes and place them in the barges, and sail by way of the Tiber to Rome; but they do not use sails or oars at all, for the boats cannot be propelled in the stream by any wind since the river winds about exceedingly and does not follow a straight course, nor can oars be employed, either, since the force of the current is always against them.
12. Instead of using such means, therefore, they fasten ropes from the barges to the necks of oxen, and so draw them just like waggons up to Rome.
13. But on the other side of the river, as one goes from the city of Ostia to Rome, the road is shut in by woods and in general lies neglected, and it is not even near the bank of the Tiber, since there is no towing of barges on that road.
14. So the Goths, finding the city at the harbour unguarded, captured it at the first onset and slew many of the Romans who lived there, and so took possession of the harbour as well as the city.
15. And they established a thousand of their number there as guards, while the remainder returned to the camps.
16. In consequence of this move it was impossible for the besieged to bring in the goods which came by sea, except by way of Ostia, a route which naturally involved great labour and danger besides.
17. For the Roman ships were not even able to put in there any longer, but they anchored at Anthium, a day's journey distant from Ostia.
18. And they found great difficulty in carrying the cargoes thence to Rome, the reason for this being the scarcity of men. For Belisarius, fearing for the fortifications of Rome, had been unable to strengthen the harbour with any garrison at all,
19. though I think that even if three hundred men had been on guard there, the barbarians would never have made an attempt on the place, which is exceedingly strong.

De Bello Gothico VI, IV
1. With these words Belisarius encouraged the Roman populace and then dismissed them; and Procopius, who wrote this history, he immediately commanded to go to Naples. For a rumour was going about that the emperor had sent an army there.
2. And he commissioned him to load as many ships as possible with grain, to gather all the soldiers who at the moment had arrived from Byzantium, or had been left about Naples in charge of horses or for any other purpose whatever - for he had heard that many such were coming to the various places in Campania - and to withdraw some of the men from the garrisons there, and then to come back with them, convoying the grain to Ostia, where the harbour of the Romans was.

De Bello Gothico VI, V
3. These [John and his troops, in Campania] set out by the coast road with the waggons, having in mind, if any hostile force should confront them, to make a circle of the waggons in the form of a stockade and thus to ward off the enemy; and they commanded the men under Paulus and Conon to sail with all speed and join them at Ostia, the harbour of Rome [The regular harbour, Portus, was held by the Goths]; and they put sufficient grain in the waggons and loaded all the ships, not only with grain, but also with wine and all kinds of provisions.

De Bello Gothico VI, VII
1. But while these negotiations were in progress at Rome, meanwhile the fleet of the Isaurians put in at the harbour of the Romans [Ostia] and John with his men came to Ostia, and not one of the enemy hindered them either while bringing their ships to land or while making their camp.
2. But in order that they might be able to pass the night safe from a sudden attack by the enemy, the Isaurians dug a deep trench close to the harbour and kept a constant guard by shifts of men, while John's soldiers made a barricade of their waggons about the camp and remained quiet.
3. And when night came on Belisarius went to Ostia with a hundred horsemen, and after telling what had taken place in the engagement and the agreement which had been made between the Romans and the Goths and otherwise encouraging them, he bade them bring their cargoes and come with all zeal to Rome. "For," he said, "I shall take care that the journey is free from danger."
4. So he himself at early dawn rode back to the city, and Antonina together with the commanders began at daybreak to consider means of transporting the cargoes.
5. But it seemed to them that the task was a hard one and beset with the greatest difficulties. For the oxen could hold out no longer, but all lay half-dead, and, furthermore, it was dangerous to travel over a rather narrow road with the waggons, and impossible to tow the barges on the river, as had formerly been the custom.
6. For the road which is on the left of the river was held by the enemy, as stated by me in the previous narrative, and not available for the use of the Romans at that time, while the road on the other side of it is altogether unused, at least that part of it which follows the river-bank.
7. They therefore selected the small boats belonging to the larger ships, put a fence of high planks around them on all sides, in order that the men on board might not be exposed to the enemy's shots, and embarked archers and sailors on them in numbers suitable for each boat.
8. And after they had loaded the boats with all the freight they could carry, they waited for a favouring wind and set sail toward Rome by the Tiber, and a portion of the army followed them along the right bank of the river to support them.
9. But they left a large number of Isaurians to guard the ships. Now where the course of the river was straight, they found no trouble in sailing, simply raising the sails of the boats; but where the stream wound about and took a course athwart the wind, and the sails received no impulse from it, the sailors had no slight toil in rowing and forcing the boats against the current.
10. As for the barbarians, they sat in their camps and had no wish to hinder their enemy, either because they were terrified at the danger, or because they thought that the Romans would never by such means succeed in bringing in any provisions, and considered it contrary to their own interest, when a matter of no consequence was involved, to frustrate their hope of the armistice which Belisarius had already promised.
11. Moreover, the Goths who were in Portus, though they could see their enemy constantly sailing by almost near enough to touch, made no move against them, but sat there wondering in amazement at the plan they had hit upon.
12. And when the Romans had made the voyage up the river many times in the same way, and had thus conveyed all the cargoes into the city without interference, the sailors took the ships and withdrew with all speed, for it was already about the time of the winter solstice; and the rest of the army entered Rome, except, indeed, that Paulus remained in Ostia with some of the Isaurians.
13. And afterwards they gave hostages to one another to secure the keeping of the armistice, the Romans giving Zeno, and the Goths Ulias, a man of no mean station, with the understanding that during three months they should make no attack upon one another, until the envoys should return from Byzantium and report the will of the emperor.
14. And even if the one side or the other should initiate offences against their opponents, the envoys were nevertheless to be returned to their own nation.
15. So the envoys of the barbarians went to Byzantium escorted by Romans, and Ildiger, the son-in-law of Antonina, came to Rome from Libya with not a few horsemen.
16. And the Goths who were holding the stronghold at Portus abandoned the place by the order of Vittigis because their supplies were exhausted, and came to the camp in obedience to his summons. Whereupon Paulus with his Isaurians came from Ostia and took possession of it and held it.
17. Now the chief reason why these barbarians were without provisions was that the Romans commanded the sea and did not allow any of the necessary supplies to be brought in to them.
18. And it was for this reason that they also abandoned at about the same time a sea-coast city of great importance, Centumcellae by name, that is, because they were short of provisions.
19. This city is large and populous, lying to the west of Rome, in Tuscany, distant from it about two hundred and eighty stades.
20. And after taking possession of it the Romans went on and extended their power still more, for they took also the town of Albani, which lies to the east of Rome, the enemy having evacuated it at that time for the same reason, and they had already surrounded the barbarians on all sides and now held them between their forces.
21. The Goths, therefore, were in a mood to break the agreement and do some harm to the Romans. So they sent envoys to Belisarius and asserted that they had been unjustly treated during a truce;
22. for when Vittigis had summoned the Goths who were in Portus to perform some service for him, Paulus and the Isaurians had seized and taken possession of the fort there for no good reason.
23. And they made this same false charge regarding Albani and Centumcellae, and threatened that, unless he should give these places back to them, they would resent it.
24. But Belisarius laughed and sent them away, saying that this charge was but a pretext, and that no one was ignorant of the reason why the Goths had abandoned these places.

Translation: Loeb, H.B. Dewing.

De Bello Gothico VII, XV, 10-12
[546 AD] So these ships were sailing toward the harbour, but the enemy spied them and got to the harbour a short time before the ships arrived; there they concealed themselves inside the walls, their purpose being that, as soon as the ships should come to the land there, they might capture them with no difficulty. And when all the men keeping guard in Portus observed this, they went up to the battlement, every man of them, and by waving their cloaks, strove to signal the men on the ships not to come ahead, but to turn aside and go elsewhere - anywhere in fact, where chance might lead them. But the men on the ships failed to comprehend what they were doing, supposing that the Romans in Portus were rejoicing and inviting them to the harbour, and since they had a favouring wind they quickly got inside the harbour.

Translation: Loeb, H.B. Dewing.