Thanks to all for the suggestions. Tonya, the schedule is great. I do see where it says Ostia Antica on the schedule, so this may work. I'll look further at the Cotral website. I hadn't had much luck earlier.
I searched around the forum and didn't find an answer to this query, so maybe I missed the earlier posting. I'll look again.
As far as luggage storage, we're heading to Ciampino after we finish with OA, so we have the option of either checking the luggage at FCO and then having to go back to collect it, or just walking around with it on our backs while at OA and then going directly to Termini from OA. Since we're flying on RyanAir and are using the one-bag approach, our convertible luggage will not be too heavy. But maybe after 4 hours of carrying it around, I may have a different opinion!
Any further tips are always appreciated.