| APA | OPK. (2012, February 10). In Acronym Geek. Retrieved 17:56, February 10, 2012, from http://www.acronymgeek.com/OPK |
| MLA | "OPK." Acronym Geek. (2012). Acronym Geek. 10 Feb 2012 <http://www.acronymgeek.com/OPK>. |
| MHRA | 'OPK', Acronym Geek, 10 February 2012, <http://www.acronymgeek.com/OPK> [accessed 10 February 2012] |
| Chicago | "OPK", http://www.acronymgeek.com/OPK (accessed February 10, 2012). |
| CBE/CSE | OPK [Internet]. Acronym Geek; 2012 Feb 10 [cited 2012 Feb 10]. Available from: http://www.acronymgeek.com/OPK. |
| Bluebook | OPK, http://www.acronymgeek.com/OPK (last visited February 10, 2012). |
| Bluebook: Harvard JOLT | See Acronym Geek, OPK, http://www.acronymgeek.com/OPK (optional description here) (as of Feb. 10, 2012, 17:56). |
| AMA | OPK. Acronym Geek. February 10, 2012, 17:56. Available at: http://www.acronymgeek.com/OPK. Accessed February 10, 2012, 17:56. |