HCO may stand for:
- Harvard College Observatory, an astronomical research organization
- Hanwell Community Observatory, a public outreach astronomy organisation
- Hearing-Carry-Over, a type of Telecommunications Relay Service
- Hubbard Communications Office, an organization in the Church of Scientology, formerly located at Saint Hill Manor, best known for issuing policy letters (e.g., Fair Game)
- Bicarbonate Hydrogen carbonate AKA: Bicarbonate
- Hollister Co., sometimes abbreviated to HCO.
- Handball Club Odense, the Danish handball club