You can pick up non zuiko wides for the OM pretty cheaply. I have a vivitar 19-35 that is quite nice, but is kinda huge using 77mm filters. 18mm is about as wide as I like to go, I actually have a 14mm sigma or tamron lens which is actually really hard to use without shooting the same cliche things everyone shoots with such a wide rectilinear lens.

Another cheaper option would be to look for wides in LTM mount and use a M adapter. The canon 28mm 2.8 is a great lens and tiny! The 3.5 is even more affordable. And if you want really wide for cheap the cv 15mm is the way to go!