A unit for measuring intensity of smell.

Initialism of Overall Position. / (theater) Initialism of opposite prompt; stage right. / (Internet) Initialism of original poster; the person who started a thread.

(fortifications) that part of the curtain whence the fire of the opposite bastion may be discovered.

(idiomatic) To conceal something about oneself, without the knowledge of which others cannot know one's true character or perspective.