This summary is not the rule. The rule text controls.
If a spot on the Board of Governors opens up before the term ends, the rest of the Board can vote to pick someone to fill that spot. This person would serve for whatever time is left in the original term. A simple majority vote is all it takes to make this appointment.
However, the Oklahoma Bar Association has some flexibility here. The Association can set up a different process for filling these vacancies through its own Bylaws. So while this rule gives the standard method, the Bylaws could spell out another way to handle it instead.