If the lens has a switch on it, you need to make sure you turn off either the in-lens OIS with the switch, or turn off IBIS (or both). Current Oly cameras don't know about OIS, and will happily shoot with both engaged, with less than desirable results.
The recently announced E-PM2 and E-PL5 cameras, however, do have a menu option to let you turn OIS on or off from the camera, so if you're using an OIS lens that doesn't have a switch you'll need to make sure you don't have both enabled in the camera menus.