onlive is really only useful for slow computers with fast connections... and mainly to play games that require Windows, so you don't need it. If its a game native on the Mac and your Mac can handle it... OnLive is just plain terrible.
If you want to game on OSX... Choose a native game if available... if you really have to play a Windows game... there are options. Below are how I'd rate them from best to worst, but opinions may vary.
1. Port the game yourself using Wineskin. Takes some learning and can be anywhere from easy to difficult... its a case by case basis. Minor to major performance hit... also a case by case basis, but usually if you get a game working decently, its pretty minor... and its totally free to use.
2. Use Crossover Games. Pretty much the same tech as Wineskin, but doesn't actually make the game its own Mac app.. it runs through Crossover, but its made for being a bit more user friendly and supported... Around the same performance as Wineskin.
3. Use Parallels or VMWare Fusion to run Windows in a Virtual Machine. You have to buy the software, and buy Windows and run a whole install of Windows. This has the least compatibility problems and you just use Windows mostly like normal, so its pretty easy. The problem is that the graphics part will take a MAJOR performance hit... and it really bogs down your whole machine.
4. Use Bootcamp to install a full version of Windows dual boot. Then you basically have a normal Windows PC... major draw backs is .. its a Windows PC now, and when your booted into Windows... you have to treat it like any other Windows machines... same +s and -s, and you cannot run OSX at the same time... meaning any multitasking you want to do all has to be done only with Windows software.