| APA | OPA. (2012, February 13). In Acronym Geek. Retrieved 23:27, February 13, 2012, from http://www.acronymgeek.com/OPA |
| MLA | "OPA." Acronym Geek. (2012). Acronym Geek. 13 Feb 2012 <http://www.acronymgeek.com/OPA>. |
| MHRA | 'OPA', Acronym Geek, 13 February 2012, <http://www.acronymgeek.com/OPA> [accessed 13 February 2012] |
| Chicago | "OPA", http://www.acronymgeek.com/OPA (accessed February 13, 2012). |
| CBE/CSE | OPA [Internet]. Acronym Geek; 2012 Feb 13 [cited 2012 Feb 13]. Available from: http://www.acronymgeek.com/OPA. |
| Bluebook | OPA, http://www.acronymgeek.com/OPA (last visited February 13, 2012). |
| Bluebook: Harvard JOLT | See Acronym Geek, OPA, http://www.acronymgeek.com/OPA (optional description here) (as of Feb. 13, 2012, 23:27). |
| AMA | OPA. Acronym Geek. February 13, 2012, 23:27. Available at: http://www.acronymgeek.com/OPA. Accessed February 13, 2012, 23:27. |