| ACO | Acceptance Checkout | In English | |
| ACO | Administrative Contracting Officer | In English | → 3474 |
| ACO | Assembly Checkout | In English | |
| AcO | Acta Oecologica | In Latin | → Serial publications, periodicals → Ecology |
| ACO | Arbeitsgemeinschaft für Chirurgische Onkologie | In German | → Associations → Oncology → Surgery, orthopaedics, ophthalmology |