Is it ensure, insure, or assure?
late 14c., "opportunity; grounds for action, state of affairs that makes something else possible; a happening, occurrence," from Old French ochaison, ocasion "cause, reason, excuse, pretext; opportunity" (13c.) or directly from Latin occasionem (nominative occasio) "opportunity, appropriate time," in Late Latin "cause," from occasum, occasus, past participle of occidere "fall down, go down," from ob "down, away" (see ob-) + cadere "to fall" (see case (n.1)). The notion is of a "falling together," or juncture, of circumstances.
mid-15c., "to bring (something) about," from occasion (n.), or else from Old French occasionner "to cause," from Medieval Latin occasionare, from Latin occasionem (see occasion (n.)). Related: Occasioned; occasioning.