means "On A Serious Note"
often used in forum threads when someone says something off topic first and then types "OASN:" with something more related to the topic following
can also be used as OMSN(On A More Serious Note), or OSN
THREAD TOPIC: FAVORITE MOVIES
my favorite movies are all the ones i stared in, directed, wrote, and produced
OASN: haha, no but really i like mostly anything made by Steven Spielberg or Michael Bay