Flash choices for use w/ OM-D
Olympus flashes will give you TTL metering (through the lens metering). Essentially the flash is automatic and decides how powerful to flash for you (the flash can talk to the camera), it does this by firing a quick pre-flash burst which the meter detects and adjusts from there (this is too fast to see). Non-Oly flashes (with the exception of Metz) will be all manual. You choose everything based on the aperture and ISO.
If you want a flashgun and aren't interested in light during video you may want to look at the FL-50r and FL-36r. I believe they also do TTL (I don't think Oly changed anything for flashes when they moved from 4/3 to m4/3). They are older but both excellent flashes.
If you're doing modelling or product photography you might want a strobe (something like an alienbee b800) instead of a flashgun (FL-300, FL-600, FL-50, FL-36). Not nearly as portable but they allow better modifiers (softboxes etc.)