Well, I found this info in an article from 2000:

*What made OM-2 so special ? Beside all those basic features which have attracted so many following worldwide, the revolutionary metering system pioneered by OM-2 meant that for the first time, a practical solution was presented to tackle metering in ambiance and flash exposure control.

Firstly, during ambient light reading in automatic mode, OM2(n)'s metering system reads light reflected from a special reflective pattern on the first shutter curtain and, during long exposures, from the film surface (thus, exposure reading may not be continuous but exposure is). This system also avoids the Asahi-patented methods using memory circuits to store the meter reading after the mirror rises (well, most SLRs during that time have their photocell(s) positioned near the viewfinder eyepiece).*The TTL OTF flash metering, in particular, opens up a lot of photographic possibilities that previously involved tedious exposure calculation. The original technology used in the OM-2, introduced more than 25 years ago, has today evolved into a mainstream flash exposure control method used virtually by all camera manufacturers.

We do not see things as they are. ǝɹɐ ǝʍ sɐ sƃuıɥʇ ǝǝs ǝʍ