Yoshihisa Maitani was a genius and came up with some fantastic cameras!

According to the May 1976 Modern Photography review of the OM2, OTF metering works different at 1/60 and above or less than 1/60. At 1/60 and above, it is off the first curtain. Below 1/60 and it is partially off the first curtain and film and will also adjust to the scene's lighting changes. I don't have a review of the OM4 so I am not certain if it continuous to work this way or not.

BTW, the only other camera that I know of that used OTF metering was the Pentax LX. This took the concept a bit further as it can meter for as long as it takes for a proper exposure or the battery dies - monitoring the scene for lighting changes and adjusting exposure in realtime.