o-1

Chemistry
1.
an abridgment of ortho-.

o-2

1.
variant of ob- before m:
omission.

o-3

1.
variant of oo-:
oidium.
British Dictionary definitions for o-

o-

prefix
1.
short for ortho- (sense 4)
o- in Medicine

o- abbr.
ortho- (often italic)