Belt keeps coming off - 1992 Oldsmobile 88

More information would have been useful - vee belt, toothed belt, multirib or flat belt and details of the application.

If the pulleys are in good condition, running true and in perfect alignment and the belt properly tensioned, it won't come off. Your belt keeps coming off so it is reasonable to assume one or more of the above is not correct.

Check all the pulleys on that belt. One is bad. Oil doesn't help, but shouldn't be the problem of the belt coming off.

Check th eidler pulley for the belt and make sure the belt is runnin in across all the pulleys in a straight line.

Could be a stretched belt, or the tensioner may not be keeping it tight.
You should have plastic panels on the bottom of the van to keep water from hitting the belt.

ur belt tension-er may be loose or worn out.

not sure what pulley you are talking about but the automatic belt tensioner may be the problem if its not keeping enough tension on the belt through out the accelerating and decelerating of the motor.this is usually the problem.

Whenever you install a new belt, it is also a good idea to replace the idler pulley.

PULLEY ALIGNMENT.WITH BELT ON LOOK AT THE PULLEYS FROM THE SIDE.U MUST HAVE A PULLEY THE IS MISSALIGNED

you probably need to chage the belt tensioner.

the only reason for the belt coming off the pulley is the something in the belt path is off aligment.
put the belt back on and chek the path where the belt pass. water pump, alternator, AC compressor, Power steeling unit, etc....... or even the pulley tensioner itself.
chance are there is a belt in the wrong pully slot.