Thank you for stopping back to tell us your choices. Nice of you. As for your cameras they certainly are different. You won't soon get bored with using either one let alone both at once. But, the Sony is just such an awesome camera. I would then bet the Olympus will fill the role of a 'back up' camera. Good shooting.
Nope, I've still got the 5D mk 3 ( it's up for sale in the ads section) but will continue to use that on occasions as well until it sells. Have been offered a trade in price against the 200-600mm Sony lens so I know what I want for it. It's still a great camera and I have managed some brilliant shots with it, but 7 years is a long time in the tech world.
I've ordered the 200-600mm Sony lens for wildlife shots but it's on back order.
The Sony should replace the old tech 5D mk3.
The Olympus is a smaller camera, with smaller lenses, should be great for when you want to be a little more discreet taking pics, like street photography or events, somewhere where a big white lens just attracts too much attention (or they won't let you in with a serious camera ;-) )
For wildlife and sport I've swapped from the D850 with 150-600mm and 70-200mm f2.8 to the Olympus EM1 Mark II with 40-150mm (80-300mm eq) f2.8 and Panny 100-400mm (200-800mm eq), and for this past weekend at the Moto GP borrowed the Olympus 300mm f4 (600mm eq).
In terms of autofocus the EM1-II is more than capable and right up there with some of the best DSLRs, only when the light is really bad does it start to struggle a touch. In terms of sharpness I can't tell any discernible difference at 'normal' viewing sizes. The area where you do see a difference is in 'pop' due to the lack of shallow DOF with m4/3 (relatively speaking). This isn't an issue for panning sports shots obviously, but if you want that pop that you get from the 70-200mm f2.8 or 600mm f4 on FF you're not going to get it. Likewise for the 'portrait' type shots you're not going to replicate that 'look' you get from an 85mm f1.8 or f1.4 on FF.
I'm really happy with the m4/3 though, having a 500g body and 900g 200-800mm eq FOV lens rather than a 1kg body and 2kg 150-600mm is a godsend. For me the weight saving more than outweighs the lack of shallow DOF. I could never afford, nor would I be willing carry a 600mm f4 lens anyway. The Olympus 300mm f4 (which gives you the look of a 600mm f8 lens on FF) is real quality and 'only' weighs 1.2kg.
Here's some of my recent efforts with the EM1-II and various tele lenses. Not the best images you'll see, but I'm no expert Generally they look better viewed on Flickr.