| ECO | Okeanis Eco Tankers Corp. | Industrials | $2.6B | — | 0 |
| OKLO | Oklo Inc. | Utilities | $6.7B | — | 0 |
| OKMN | Okmin Resources, Inc. | Energy | $6.5M | — | 0 |
| OKTA | Okta, Inc. | Technology | $34.3B | — | 0 |
| OKYO | OKYO Pharma Limited | Healthcare | $52.2M | — | 0 |
| ODFL | Old Dominion Freight Line, Inc. | Industrials | $36.4B | — | 0 |
| OMCC | Old Market Capital Corporation | Communication Services | $28.3M | — | 0 |
| ONBPP | Old National Bancorp | Financial Services | $10.3B | — | 0 |
| ORI | Old Republic International Corporation | Financial Services | $9.2B | — | 0 |
| OSBC | Old Second Bancorp, Inc. | Financial Services | $1.3B | — | 0 |
| OLMA | Olema Pharmaceuticals, Inc. | Healthcare | $790M | — | 0 |
| OLOX | Olenox Industries Inc | Industrials | $0.5M | — | 0 |
| OLN | Olin Corporation | Basic Materials | $1.9B | — | 0 |
| OLLI | Ollie's Bargain Outlet Holdings, Inc. | Consumer Defensive | $5.1B | — | 0 |
| OMDA | Omada Health | Healthcare | $1.2B | — | 0 |
| OMGGF | Omai Gold Mines Corp. | Basic Materials | $1.2B | — | 0 |
| OFLX | Omega Flex, Inc. | Industrials | $254M | — | 0 |
| OHI | Omega Healthcare Investors, Inc. | Real Estate | $13.9B | — | 0 |
| OMER | Omeros Corporation | Healthcare | $1.5B | — | 0 |
| OABIW | OmniAb, Inc. | Healthcare | $1.2B | — | 0 |
| OMCL | Omnicell, Inc. | Healthcare | $1.5B | — | 0 |
| OMC | Omnicom Group Inc. | Communication Services | $20.5B | — | 0 |
| OMQS | OMNIQ Corp. | Technology | $1.1M | — | 0 |
| OMTK | Omnitek Engineering Corp. | Consumer Cyclical | $0.2M | — | 0 |
| OMSE | OMS Energy Technologies Inc. | Energy | $179M | — | 0 |