After a week away using the Olympus OM-1 and Olympus 100-400mm lens in anger I came away not happy with the IS for video. Lots of micro jerks all over the place ruining the footage.
So I carried out a direct comparison to my old Panasonic GH5 and Panasonic 100-400mm lens.
All shots were at 400mm.
The conclusion is it looks like you can workaround the Olympus issue by turning the body IS off and letting the lens just do the IS. I would have thought the Olympus setup, since the camera is a few months since release and the lens is about a year old, would at least match the old GH5 gear but seems not if you leave IS on both like you would on the Panasonic.
I think the workaround for the Olympus gear gets it good enough. It certainly gets rid of the micro jerks.