From my Local Ag Extension Office:
Do you have a herbicide license? If so Tordon 22K or Surmount will do a good job of controlling the prickly pear but do not use these under live oaks. If you not have a license PastureGard HL does a great job of controlling prickly pear and is safe to use under oak trees. Any of these prickly pear herbicides need to use at a rate of 1.00% (1.28 oz/gallon of water) mixed with a surfactant plus blue dye, the prickly pear needs to be sprayed on both sides of the pads and all three herbicides will not harm the grasses.
PastureGard is not the cheapest stuff but worked extremely well-within 3 months could see results and what was sprayed last year is all ready white and decaying. It's important that you take the time to spray both sides of the pads to ensure a good kill