Suggest that you disconnet the wire from the OP sender and than using a good multimeter, see if you get continuity from ground to terminal (make sure it is not running). If you do then, more than likely the switch is good for when it is not running, it should ground completing the circuit and causing the light to come on. If you do get continuity when it is not running, start her up but leave the OP wire disconnected but not touching anything, check the continuity from the OP Sender terminal to ground again. If you do not have continuity this time (Open circuit), the OP switch is good.
If I understand, the light is not coming on at all. So if the switch is good, than more than likely, have a broken wire, bad connector or possibly a broken connection on the tach ring.
Have you had your EHC disconnected recently? I broke a pin on one of mine awhile back and it was for the OP light.