Ostia Antica and catacombs in one day

Yes, spend a half day at each. Actually tour of a catacomb is much shorter but you may want to wonder a bit along the A Way. Sunday is a better day to see the A way because most (not all) of the vehicle is restricted. OA can take longer simply because it is fairly vast but a good two to three would be adequate to see the highlights. All are reachable via public transit.

Both these sights can be easily done in a day. (We did the Catacombs in the morning and the Forum and Colloseum in the afternoon and coupled Ostia Antica with St Peter's on another day) We spent about 2 hours at the Catacombs and 3 1/2 hours at Ostia Antica. My best tip for Ostia Antica: When you ride the train down, you enter through the "back door". Walk all the way to the front (do not pass go, do not collect 200 dollars) and start at the visitor's center. Than work your way back to where you entered enjoying the sights. We spent a lot of time in the back not realizing most of the "best stuff" was up front and were getting tired by the time we got there. Have fun.

As for transportation: You will use the bus to get to the catacombs. You can ride the Metro to the Circo Massimo exit and catch the bus (going past the Baths of Caracilla). You will have to ride back to the Circo Massimo area and then go south to the Pyramide area to catch the train to Ostia Antica. (You can catch a bus or go one stop on the Metro, or even walk-although you may want to conserve energy for Ostia Antica) If you haven't already, you should look into a Roma Pass and see if it will pay for you to purchase. (Includes 3 day consecutive days on buses and Metro and includes the train to Ostia Antica) If you don't get that, you probably should purchase at least a one day pass for transportation for this day.