Largely agree with the above. I'm sure the majority of photographers, who do not need the ultimate "toughness" of the Nikon brand will be perfectly happy with the OM series. When the Nikon F was first produced, it was aimed at pros. Nikon did their homework and they realised that what professionals (particularly those involved with news photography) demanded from their cameras was the ultimate in reliability, which meant the ability of the camera to withstand every kind of abuse and yet carry on working in all weather conditions. Consequently elements of the "tank like" build quality have filtered down to their later FM2's & FE2's as well.
Today the demands (even from pros) are different. In a media world that is now celebrity driven rather than news driven, they want maximum megapixels. Toughness of build is no longer the primary requirement.
Back in the 70's & 80's when I was active as a pro., the OM series were popular for a while and were regarded as being nice cameras for feature work, (for instance they were popular with fashion photographers) but lacking the ultimate strength to take the battering from day to day news photography in all kinds of weather. Nikon and Canon with its F1 were favoured here. Yes they tended to be on the heavy side and not particularly compact, the penalty paid for that extra sturdiness.