This isn't the best site for discussion of Olympus or M4/3, and you would better be asking questions in a specialist forum. In case you don't get any replies, here is the personal view of a birder who shoots Canon.
I've never used Olympus but have thought carefully about adding one for many years for my birding photography with telephotos, but I always decide against. The OM-1 and the new 150-400mm f4.5 TC1.25x IS PRO Lens are superb pieces of kit and make a great stand-alone system but they don't complement or really add to what I have already; R7, R5, RF 100-400mm and RF100-500mm.
Firstly, in terms of reach, the R7 and OM-1 have virtually the same pixel densities, equivalent to ~80 Mpx FF. The 150-400mm with 1.25xTC kicked in has f/5.6, only 2/3rds of a stop faster at 500mm than the 100-500mm at f/7.1. Not only is that a small difference, it's neutralised by the larger sensor of the R7 that is worth 2/3rds of a stop!
Secondly, when it comes to BIF, the R5 is ranked higher than the OM-1 on the site that measures these, and more importantly, I couldn't handle 400mm on a M4/3, which is equivalent to 800mm field of view on my R5 for fast birds. I would have to zoom out to 250-300mm, and this would more than negate any reach advantage - 250mm on a 20Mpx M4/3 sensor has the same fov as 500mm on a 45 Mpx R5, but only the reach of 326mm. And, the wider f/4.5 is negated by the smaller sensor.
Other people may have different views and the OM-1 is the best for them, but not for me with my current Canon gear. And, if a higher resolution R5 comes out, it will be even less so.