Apparently it takes 3/4 which in OK's house would be 36-12, but they did today override a veto on an anti-abortion bill, which shows that there are enough senators more conservative than the governor to override him on at least some issues, and willing to do so via a veto as well. Sometimes legislators will vote for a bill but not for a veto override, but apparently they have enough at the moment if the issue is right.
The bill originally passed the Oklahoma House: 81-14 and the Senate 39-3. It looks like an override is possible.