A small opto isolator isn't going to save you from lightning. It should protect from all the
pickup induced on the wires, but provide a good earthing stake outside the building directly
bonded to the wire.
Lightning jumps many feet at a time (stepped leader), you have to provide a solid
alternate route to ground for the current and perhaps a metre or more of isolation to
have a chance against a direct hit to the cabling:
Using a length of optical fibre / lightpipe to isolate, bright LED at one end, good photo
transistor at the other (you can get black plastic coated light pipe to keep ambient light out) - its like an opto-isolator but can be several metres long.
I suspect someone has already investigated such a lightning-proof opto-isolation set-up
before, google around maybe?