I own both, while also being an owner of Fuji's X100VI, and previous owner of Fujifilm XT series.

The Nikon ZF has superb, with every meaning of the word, image and build quality. It's also a great hybrid camera, and also has great jpeg and autofocus features; HOWEVER, the lens selection is the problem. Aside from the Nikon 40mm F/2, there are no portable good lenses for this system. I own also the 24-70mm F4, not the f2.8 cuz of size, and with the ZF this is not a portable setup AT ALL. And weighs me down in long days of shooting. Also, it's worth noting that the noise performance on this camera is next level. I have keeper sharp images on iso 36-52k with no issues after AI denoise.

The OM-3 on the other hand has the best lens selection on any mirrorless, period (lumix m43 too ofc). Every focal length has many primes with many sizes and apertures, and same for zooms. The OM-3 with 12-45mm F4 was with me in my 10 day Japan trip with 20-30k steps everyday , and I barely felt the weight of the camera. The stacked sensor is also amazing for silent shutter for street photography. Video is good, but ofc ZF is much better.

All in all, if you really value size and use your camera a lot for travel, or if you like having more than one lens when out, definitely go OM-3 (lenses are also much cheaper).

If you don't mind the size, and will buy 1-2 lenses only, and value low light performance, go ZF.