Have a look at
http://www.bhphotovideo.com/c/produc...ght_Angle.html
I don't know if this is the same one that fits on a OM-1 - you'll need to find out.
More info on the OM-1/2/3/4 one at
http://www.alanwood.net/photography/...ni-finder.html
You can also get focussing screens from B&W (and probably elswhere too), although the cost of buying a new one is probably more than the OM-1 is worth.... It might be cheaper to look for a second hand camera with the screen already in it.
http://www.bhphotovideo.com/c/shop/2...0/mxp/0.0/pn/2