almost half the price
image size is a much more usable and workable 20mp for most scenarios
the Sony MUST be shot at 61mp when in full frame mode which results in gigantic file sizes which become far larger when post-processing into TIFFs or PSD or similar files to the point that post-processing becomes a chore even on reasonably powerful laptops with 16Gb RAM. Of course, you could re-size these to 20mp TIFFs but these are far larger than 20mp Olympus RAW files as they are 16bit vs 12bit.
IBIS is significantly better
weathersealing is almost certainly much more reliable
rear screen is fully articulating rather than tilt only and is far more functional
2×2 switch allows more rapid access to different settings
telephoto lenses are shorter for the same telephoto reach
HiRes mode is far more useful:
50mp high resolution is probably all that anyone really needs
HiRes mode can be done hand held
tripod HiRes mode further improves image quality
Olympus HiRes will have far less moire than the Sony 61mp normal mode so will be of more benefit in fashion or product photography in particular
faster sensor readout and less rolling shutter which allows:
better sensor dust mechanism
a better PASM dial
two extra buttons on front of camera
Pro-Capture mode
Focus bracketing and in-camera Focus Stacking modes
night Live Composite mode and Live BULB, Live Time modes
EYE AF allows selection of closest eye as an option
in camera focus range limiter that works on any MFT lens
Starry Sky AF mode
marginally better menu system
able to save RAW+jpeg to one card and only RAW to another card
easier switching of cards in playback
it is more fun to use and to post-process