Nervy, Rotorheads is aimed at pros (it's in the forum description). Therefore they don't have to speak English; they can talk shop.
I'm not a pro myself (in fact I'm barely even an amateur), but I read all the threads and try to work out as many things for myself as I can. It helps that I subscribe to JAR-OPS3 (the manual that covers AOC Helicopter operations); and you can bet the pros (certainly in Europe) have very easy access to that.
In this case, OEI is 'One Engine Inoperative'. By contrast, AEO is 'All Engines Operating'.