I was really tempted to purchase an OM-1 with the OLY 100-400 using their new promotion for $400 off plus trade-in of old gear....but...I realized that the Oly zoom lenses zoom the opposite way of my Nikon and Sony zoom lenses. It may sound crazy, but I think it's a deal breaker for me and that system. I have the OM-5 and OM-10, but I've only really used prime lenses. I've used Panasonic Zooms (they turn the same way as Nikon and Sony), but Ultimately I'd really want the Oly 100-400 or the 150-400 for their compatibility with Oly body and the Price of the 100-400.
I just don't want to mess up my muscle memory.
I switched without difficulty. There issues going from DSLR (Nikon D-500) to mirrorless with the OM-1 because you need to pay particular attention to proper exposure, and you really need to learn to trust the Subject ID AI as opposed to placing the FP but the zoom direction issue was in the noise for me.
That $400 off deal drops the price of an OM-1/100-400 to an insanely low level for a modern BSI stacked sensor setup. We due respect to Nimi's point that the long-term development future of the m4/3 sensor is bleak, that is still one awesome deal.