A fat zener or transorb across the opto input won't protect it from overvoltage. It will only protect from reverse bias.

Try adding a 100 ohm resistor and 4.7V zener diode like such:

               11k           100     LED
 +110VDC o----///----.----///----|>|----.----o GND
                       |                    |
                       |         /          |
                       '---------|<|--------'
                                 /
                               4.7V Zener

Don't worry about the size of the zener - 1/2W would be more than enough. The larger resistor will by far dissipate the most power during a transient overvoltage.

- Lee