I have a Canon EOS Rebel T3/1100D and want to use an old lens. What kind of adapter should I use for an Olympus Zuiko AUTO-Zoom 75-100mm f/4.0 lens?
That's an Olympus OM mount lens and the T3 uses the Canon EOS mount, so you need an OM -> EOS adapter ring. You will want to make sure the ring has a stop-down pin on it to keep the DoF preview lever on the lens depressed, so the lens will stop down as you adjust the aperture ring.
Be aware that you will not have aperture control from the camera (you have to adjust aperture with the lens ring), you have to use stop-down metering, there's no autofocus, and you'll be limited to the M or Av shooting modes on the camera because the camera can't control the aperture of the lens. In addition, none of the lens EXIF information will be transmitted, unless you get a chipped adapter ring.
I'd also suggest finding a reputable repair person and budgeting for a CLA, depending on the condition of the lens you've got.