| Subject | English | Thesaurus |
| abbr., med.appl. | B.O.L. | beginning of life |
| tech., abbr. | CO2-D2O reactor | CDR |
| abbr., med.appl. | E.O.L. | end of live (of a pacemaker) |
| abbr., telecom. | I/O | Input (voltage earth), Output |
| water.suppl. | O & M program | operation and maintenance program |
| tradem., abbr. | O'Brien Investment Services | OIS |
| abbr., lat. | O.CD. | Ordo Carmelitarum Discalceatorum (Discalced, or Barefoot, Carmelites) |
| abbr., mil., avia. | O&CO | operations and checkout |
| abbr. | O.F.M.Cap. | Ordo Minorum Cappucinorum (Capuchins) |
| abbr., telecom. | O&M center | operation and maintenance center |
| lat. | O.M.Cap. | Ordo Minorum Cappucinorum (Capuchins) |
| abbr., el. | O-POS | oxygen-doped polysilicon |
| abbr., file.ext. | Program Controlled I/O | PC-I/O |
| telecom., abbr. | Program Controlled I/O | PCIO |
| Nasdaq, abbr. | S O S Staffing Services, Inc. | SOSS |
| invest. | SE&O | salve errore et omissione |
| abbr., mil., WMD | SFAE-CD-CO-O | Office symbol for PMCD operations branch |
