I shall be in Rome in May and have 3 questions about sightseeing
1. Are the fountains in Tivoli working at present? Do the fountains work every day? Is it necessary to reserve a tour ahead of time?
2. What HOHO bus do you recommend?
3. Is it necessary to reserve a tour to Ostia Antica ahead of our arrival in Rome?

You can take the Lido train from Stazione Porta San Paolo (Metro Piramideto Ostia)to station Ostia Antica (I think they run every 15 min. or so) and do a self-guided tour - most guidebooks have them.

SS

If you are talking about the fountains in Villa d'Est, in mid-March, only couple major ones were working. There might be more in May. No tour reservation necessary, as it's easy to take bus to Tivoli from Rome.

For Ostia, the best advice is to not start at the entry, but instead to walk first all the way to the back, without stopping (which is hard to do) and then tour from the back to the front. The reason is that many of the important buildings are in the back area and many people miss them because they get tired and spend too much time near the entry.

I highly recommend that you stop at the train station for St. Paul's Outside the Wall on the way to Ostia Antica. I was surprised to find that it's one of my favorite churches in Europe and believe me, I've seen more than I could ever count.

It's an easy departure, you can spend an hour or so and walk back to continue the train trip to Ostia. The walk is maybe 15 minutes if that.

I forgot to say that we did not get a tour of Ostia Antica but there are descriptive markers along the way. I found that quite satisfying. We literally strolled up to the top, taking pictures along the way. But, do plan on a whole day or at least most of it.