- O Elvas (Football)
- O'Connor Knights FC (Football)
- O'Shaquie Foster (Boxing)
- Oakland (Basketball)
- Oakland Athletics (Baseball)
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- OB (Football)
- Odd (Football)
- Odisha (Cricket)
- Odisha Women (Cricket)
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- Offaly (Gaelic Football)
- OFI (Football)
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- Ohio (American Football)
- Ohio (Basketball)
- Ohio State (Basketball)
- Ohio State (American Football)
- Oil & Gas Development Company Limited (Cricket)
- Oklahoma (Basketball)
- Oklahoma (American Football)
- Oklahoma City Thunder (Basketball)
- Oklahoma State (American Football)
- Oklahoma State (Basketball)
- Olanrewaju Durodola (Boxing)
- Old DOHS Sports Club (Cricket)
- Old Dominion (American Football)
- Old Glory DC (Rugby Union)
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- Oldham RLFC (Rugby League)
- Ole Miss (American Football)
- Ole Miss (Basketball)
- Oleksandr Gvozdyk (Boxing)
- Oleksandr Usyk (Boxing)
- Olimpia (Football)
- Olimpia Milano (Basketball)
- Olimpija Ljubljana (Football)
- Oliveira do Hospital (Football)
- Olympiacos FC (Football)
- Olympiacos Piraeus (Basketball)
- Olympic FC (Football)
- Olympic Kingsway SC (Football)
- Omaha (Basketball)
- Oman (Cricket)
- Oman (Football)
- Oman A (Cricket)
- Oman Under-19s (Cricket)
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- Omar Douglas (Boxing)
- Omonia Nicosia (Football)
- Ondrej Pala (Boxing)
- Oral Roberts (Basketball)
- Ordabasy (Football)
- Oregon (Basketball)
- Oregon (American Football)
- Oregon State (American Football)
- Oregon State (Basketball)
- Orlando City SC (Football)
- Orlando Guardians (American Football)
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- Osasuna (Football)
- Oscar De La Hoya (Boxing)
- Oscar Duarte (Boxing)
- Oscar Rivas (Boxing)
- Oscar Valdez (Boxing)
- Oskari Metz (Boxing)
- Oskarshamns AIK (Football)
- Osmanlispor FK (Football)
- Ospreys (Rugby Union)
- Ossudu Accord Warriors (Cricket)
- Otabek Kholmatov (Boxing)
- Otago (Rugby Union)
- Otago (Cricket)
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- Ottawa (Ice Hockey)
- Ottawa Charge (Ice Hockey)
- Ottawa Rapid FC (Football)
- Ottawa Redblacks (Canadian Football)
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- Oxford City (Football)
- Oxford United (Football)
- Oxford United Women (Football)
- Oxford University (Cricket)
- Oxfordshire (Cricket)
- Oxfordshire Women (Cricket)
- Oyonnax Rugby (Rugby Union)