I'm a Nikon guy, so not an expert on Olympus. BUT, in general you want to buy a lens with the biggest glass (aperture) possible, f2.8 or smaller (f2.0, f1.8, etc).
Olympus 18-180mm f/3.5-6.3 zoom that is both wide angle and long, but shooting at f6.3 will only work in bright sunlight. A cloudy...