On an OM-10, the self timer immediately triggers the shutter instead of waiting 12 seconds. Sometimes the camera opens the shutter (like it would be on B position) then beeps for two second and closes the shutter. Apart from this other features work well.
First test: take lens off, wind shutter, then carefully lift edge of mirror up. Does mirror hit a stop about halfway up? If not, the mirror magnet is not working. Without this, self timer cannot hold the mirror partway for it's delay. If the mirror does stop halfway, try rocking the selector switch back and forth. This may clean the self timer contacts temporarily.contributed by John Hermanson