The current Sigma 15mm f/2.8 fisheye I think has a RRP of about £700 but I would think you could get this a lot cheaper shopping around.
My only concern would be the fact that current independent lenses are normally designed for digital SLR's, so you may have some compatability issues with your OM-1. The lens should mount on the camera OK, but you'd have to check before buying to see whether, for example, it had an aperture ring as you'll be using this on a fully manual camera.