Welcome to the Friendliest Board This Side of Riedquat, Commander.
You can use as many OXPs you wish, the limit being your machine's processing power; they are almost all compatible, or have a caveat about which other OXPs they are incompatible with (unless you're the first to discover a problem, but that's another matter). The exception are the HUDs, which you can only have one at a time.
It can also happen that the OXP not only isn't incompatible, but it actually needs others installed to work at all, and some few OXPs are even configurable - but don't worry, you'll find more about these issues on each OXP's wiki article or in the readme files.
To install your OXPs, you have to find the folder OXPNAME.oxp inside the downloaded file, and drop it in the folder Oolite\AddOns.
looking forward to regaining my Elite status..... damn the 80's were good!
Have fun, Commander. The Oolite community is always pushing the envelope, trying to allow in this game all we ever wanted to see in Elite (Generation Ships already exist in an OXP; I don't doubt eventually someone will do one about finding Raxxla) and much, much more...
You know those who, having been mugged and stabbed, fired, dog run over, house burned down, wife eloped with best friend, daughters becoming prostitutes and their countries invaded - still say that "all is well"?
I'm obviously not one of them.