Remember that you are buying into a system. Olympus requires olympus mount lenses. Same for Nikon, Canon, Sony, etc.

Third party lenses vary in quality. I have a Tokina 12-24 that I'll put up against Nikon's 12-24 and you'll need a magnifying glass to tell the difference. The Tokina is half the price of the Nikon.
Only way to know is read lots of reviews on the lenses. The good thing is that third party lenses are made to mount on all the manufacturers cameras and are much less expensive.