Below is a general plan of Ostia with indications of the five modern regions of the site.
Clicking on a region will take you to a colour-coded plan of that region: clickable, but low resolution.
Note 1. The full colour-coded plan is here (not clickable, low resolution).
Note 2. On the pages-per-region you will find links to details of the colour-coded plan (not clickable, high resolution).