Dictionary.com
definitions

o-

1
Chemistry.
  1. an abridgment of ortho-.

o-

2
  1. variant of ob- before m: omission.

o-

3
  1. variant of oo-: oidium.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
British Dictionary definitions for o-

o-

prefix
  1. short for ortho- (def. 4)
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
o- in Medicine

o-

abbr.
  1. ortho- (often italic)
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.