OASN 

On a Sober Note. Something you say to a good friend to indicate that you are really, really serious about the topic at hand and your word should be taken as if you were sober even though you are drunk as shit and have been talking out of your ass for the better part of the evening.

No, man, no! Listen! OASN - I lover her!

OASN by MJohnsonD August 17, 2016

OASN 

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

OASN by Nosredna RM December 23, 2009