What is the distance and travel time, and do city buses travel to this port?
The posts on this thread give information about train travel rather than bus travel, but they should be of help to you.
Maybe if you provided a little information we could give you a better answer. Ostia is kind of an odd place to be coming from and I am sure there is no direct bus link to Civit...
We were planning a transfer to our cruise ship by taxi, but thought there might be a cheaper way such as bus. Any thoughts?