OP - "Ordo Praedicatorum" (Latin for "Order of Preachers"), religious order founded by St. Dominic de Guzman (1170 - 1221).

Its white-clad (sometimes wearing a black cape) members are better known as the Dominicans. Their motto is Contemplata aliis tradere, which roughly translates "to teach fruits of contemplation". Their most famous member was Thomas Aquinas, OP. Indeed, in modern days, all Dominican priests take a vow of only teaching theology based on Aquinas.

Historically, they were in charge of Holy Inquisition.