For the most part, these are synonymous and can be used interchangeably. However there are some minor nuanced differences when you look at their definitions.
OP - OverPowered, refers to a large unmitigated power gap when compared side-by-side with other characters, abilities, items, etc.
Broken - something that literally "breaks" the game with its presence, that solely defines or alters the meta to play with/around it.
Imba - Imbalanced, a distinct distortion away from typical game balance that is noticeably outside the margin of error.
Level 16 Ranger - Please just disable this.(Reserved solely for Deadpool)
PS: Kudos to anyone that gets that last reference. RIP MAA.