The first letters are the type of signal received:
O=Flarm
P=PilotAware
M=MLAT (provided by 360radar)
A=ADSB
a=Cap1391
The final letters are if received locally or remotely.
l=received locally
r=received remotely
There are possible combinations:
e.g.
OPA = aircraft is transmitting Flarm, PilotAware and ADSB.
PM = PilotAware and the Mode S is MLATd.
etc