| Name | What | Region | Country | Lat | Long | Elev ft. | Pop est |
|---|---|---|---|---|---|---|---|
| O-Pojana | city | Harghita | Romania | 46.95 | 25.77 | 3349 | 1391 |
| Obrăneşti | city | Harghita | Romania | 46.22 | 25.28 | 2509 | 3614 |
| Ocfalva | city | Harghita | Romania | 46.25 | 25.25 | 1719 | 12148 |
| Ocland | city | Harghita | Romania | 46.17 | 25.42 | 1669 | 1966 |
| Ocna de Jos | city | Harghita | Romania | 46.53 | 25.13 | 1669 | 3399 |
| Ocna de Sus | city | Harghita | Romania | 46.53 | 25.15 | 2509 | 4404 |
| Oczfalva | city | Harghita | Romania | 46.25 | 25.25 | 1719 | 12148 |
| Oderhellen | city | Harghita | Romania | 46.30 | 25.30 | 1709 | 34638 |
| Odorhei | city | Harghita | Romania | 46.30 | 25.30 | 1709 | 34638 |
| Odorheiu | city | Harghita | Romania | 46.30 | 25.30 | 1709 | 34638 |
| Odorheiu Secuiesc | city | Harghita | Romania | 46.30 | 25.30 | 1709 | 34638 |
| Odorheiul Secuiesc | city | Harghita | Romania | 46.30 | 25.30 | 1709 | 34638 |
| Odorheju Sekujeskas | city | Harghita | Romania | 46.30 | 25.30 | 1709 | 34638 |
| Okland | city | Harghita | Romania | 46.17 | 25.42 | 1669 | 1966 |
| Oláh-Andrásfalva | city | Harghita | Romania | 46.30 | 24.93 | 1669 | 3611 |
| Oláhfalva | city | Harghita | Romania | 46.35 | 25.52 | 2713 | 4242 |
| Oppidum Mercurium | city | Harghita | Romania | 46.35 | 25.80 | 2509 | 33465 |
| Opština Borsec | city | Harghita | Romania | 46.95 | 25.57 | 3736 | 2434 |
| Opština Cristuru Secuiesc | city | Harghita | Romania | 46.28 | 25.03 | 1669 | 10931 |
| Opština Gheorgheni | city | Harghita | Romania | 46.72 | 25.62 | 2811 | 16645 |
| Opština Odorheiu Secuiesc | city | Harghita | Romania | 46.30 | 25.30 | 1709 | 34638 |
| Oraseni | city | Harghita | Romania | 46.17 | 25.35 | 1669 | 1829 |
| Oroszhegy | city | Harghita | Romania | 46.40 | 25.30 | 2509 | 4057 |
| Orotva | city | Harghita | Romania | 46.87 | 25.47 | 2559 | 4787 |
| Oteni | city | Harghita | Romania | 46.25 | 25.25 | 1719 | 12148 |