I have a Microsoft Wireless Comfort Keyboard 5000 with pre-set short cut keys including F3 already assigned as Undo.  Is there another short cut key or command I could use to quickly turn on and off o snaps?

Open your CUIx and see what, if any, function key might be available.  Maybe F12?  Otherwise maybe you can substitute the OSnap command for one of the function keys you don't use.  I'm not sure but I think any other key to enable OSnaps may end up having to be a key-combo (two keys) utilizing ALT or SHIFT or CTRL.