O-sensei is the title of respect given to morihei ueshiba, founder of Aikido. Sensei is the Japanese word for teacher.

In japanese, the prefix "O" means "honored" or "respected" (or sometimes just "big"), so "O'Sensei" is, essentially, "Honored Teacher" (where sensei already implies a certain amount of honor); essentially, the "first teacher" or "most honored teacher".