Early FCO flight...should I spend time in Ostia Anitca?

I would go to Pompeii on the 10/17 on the way to Naples. Then in the afternoon or evening, I'd head to Rome from Naples (70 min train journey). You can stay in a hotel in the city center of Rome and spend in Rome the rest of the evening. There are lots of things to do in Rome in the evening.
Then in the morning, you take a taxi (or the Leonardo Express train) to FCO for your flight home. The travel time is only about 30 min, so you can comfortably leave from Rome city center at about 7, and be at the airport well ahead of the two hours required for a US bound flight.
If you absolutely want to visit Ostia Antica, then the day before (10/17) you could leave Naples at about lunch time (after seeing Pompeii in the AM) and get to Rome in early PM. Leave your stuff at the hotel and head to Ostia Antica. You may not have a lot of time however. The site last entry in October is at 5:30pm (closing time is at 6:30pm).
Try to stay at a hotel near the Termini station, so that you minimize the time to get to the station from the hotel, especially if you plan to take the Leonardo Express to FCO in the morning.

I think of Ostia as the "poor man's alternative" to Pompeii. If one is in Rome, it's an interesting site and and short distance from the city. I would go to the National Archaeology Museum in Naples instead - it's a wow. From the Circumvesuviana train in Naples, go up to the main train station to store your luggage, then take the metro to the museum and back and get a late train to Rome, stay near Termini. There are wooden storage lockers at the museum, but they may not be big enough for your luggage. Looks like the last fast train leaves Naples at 7.30 pm. Plenty of time to enjoy Pompeii and the museum without rushing.