| # | Company name | Role | # of projects |
|---|---|---|---|
| 1 | O’Halloran & O’Brien | Construction company | 4 |
| 2 | OAC Action Construction, Corp. | Construction company | 0 |
| 3 | OAO Stroyindustria | Construction company | 9 |
| 4 | OBAS AS | Construction company | 0 |
| 5 | OBAS Vest AS | Construction company | 0 |
| 6 | Obayashi Corporation | Construction company | 703 |
| 7 | Obayashi Corporation (Malaysia) Sdn Bhd | Construction company | 4 |
| 8 | Obraprima | Construction company | 2 |
| 9 | Obras & Sistemas S.R.L. | Construction company | 9 |
| 10 | Obras Mayve | Construction company | 7 |
| 11 | Obrum | Construction company | 0 |
| 12 | OCEA | Construction company | 0 |
| 13 | Ocean Avenue Construction | Construction company | 2 |
| 14 | Odakyu Construction | Construction company | 17 |
| 15 | Odebrecht Engenharia e Construção | Construction company | 45 |
| 16 | Oder Construction Ltd | Construction company | 1 |
| 17 | Oetken GmbH & Co. KG | Construction company | 0 |
| 18 | Oevermann GmbH & Co. KG | Construction company | 5 |
| 19 | Ofek Bublil Ltd. | Construction company | 7 |
| 20 | Ofton İnşaat | Construction company | 1 |