Can UO be played on a mac? if so how?
Yes
With a copy of windows:
Bootcamp, Parallels, one or two others?
Without a copy of Windows (these solutions implement the windows API for the Mac)
Crossover & Wine
The easiest solution is crossover ($30) from code weavers. A 30 day trial is downloadable from the codeweavers site. I've never set up Wine (FREE!) for a Mac but it was really easy for Linux; though it requires you install the X11 package on the Mac.
Bootcamp comes with MacOS X but required you to boot in to Windows. The others can be run from a Mac desktop.