What does op mean on Tumblr?

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Tags

1. #context
2. #occasionally
3. #sharing
4. #poster
5. #blogging
6. #published
7. #refers
8. #originally
9. #although
10. #words
11. #focusing
12. #question
13. #content
14. #simplicity
15. #platform
16. #tumblr
17. #primarily
18. #other
19. #original