I've been to Ostia several times between 2003 and 2005 but found the museum closed on every occasion. There seems to be a distinct lack of funding, which is also visible in the preservation of the city. That said, it would be a pretty daunting task to pull off, especially given the limited number of visitors. It's a real shame, because Ostia ranks right up there with the best Italy has to offer.
So, although things may have changed in the mean time, you'd better bring along all the information you might need. The mosaics are preserved in situ. And as I said before, bring a good, detailed map. There are many hard to find gems (like the mithraeums, synagogue, imperial palace) that you're likely to miss otherwise.
If you go on a week day, you'll have the place to yourself. If the weather is decent, the Romans will come down from the city in droves to have a picnic in the weekends. Also, check the opening hours: they are usually changed from summer to winter schedule in October. And make sure you're at the gate when it opens, because you'll need a full day to see it all.
Enjoy your trip!