| APA | OPO. (2012, February 13). In Acronym Geek. Retrieved 23:26, February 13, 2012, from http://www.acronymgeek.com/OPO |
| MLA | "OPO." Acronym Geek. (2012). Acronym Geek. 13 Feb 2012 <http://www.acronymgeek.com/OPO>. |
| MHRA | 'OPO', Acronym Geek, 13 February 2012, <http://www.acronymgeek.com/OPO> [accessed 13 February 2012] |
| Chicago | "OPO", http://www.acronymgeek.com/OPO (accessed February 13, 2012). |
| CBE/CSE | OPO [Internet]. Acronym Geek; 2012 Feb 13 [cited 2012 Feb 13]. Available from: http://www.acronymgeek.com/OPO. |
| Bluebook | OPO, http://www.acronymgeek.com/OPO (last visited February 13, 2012). |
| Bluebook: Harvard JOLT | See Acronym Geek, OPO, http://www.acronymgeek.com/OPO (optional description here) (as of Feb. 13, 2012, 23:26). |
| AMA | OPO. Acronym Geek. February 13, 2012, 23:26. Available at: http://www.acronymgeek.com/OPO. Accessed February 13, 2012, 23:26. |