1. Symbol for oxygen; orotidine.
2. Abbreviation for opening (in formulas for electrical reactions).
3. Symbol for a blood group in the ABO system. See ABO blood group, in Blood Groups Appendix.
4. An abbreviation derived from the German phrase ohne Hauch (without film), used as a designation for: antigens that occur in the bacterial cell, in contrast to those in the flagella; specific antibodies for such somatic antigens; the agglutinative reaction between somatic antigen and its antibody.

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