obvi
off the hook 
off the top of my head
OD 
OK Boomer!
okie dokie 
okurrr
OIC 
old fart 
old school
OMG 
once in a blue moon 
on fleek 
on point
one-hit-wonder 
one love 
one night stand 
one way or another 
OOMF 【ベストスラング95位】
OOTD/OOTN 
oops 
or nah / or naw
OTP
OTT 
out of one's league　
out of shape [2021]
outie 
overstand