In the context of an Oklahoma divorce, joint custody is an award by a court of legal custody of a child to more than one person, most typically to both parents of a child. Legal custody of a child is the right to make legal decisions regarding the care and custody of the child.
A shorthand I use sometimes to characterize a custody decision is that any decision that is still affecting the child long after it is made is probably a custody decision. So what kind of medical treatment the child receives and what school or type of education the child attends or receives are good examples of a custody decision. A parent with legal custody ordinarily has the right to determine these things within certain bounds (essentially legality, a custodial parent does not have the right to commit child abuse or expose the child to drugs, for example).
When more than one person exercises joint custody there is typically a decision making process set forth by the court order granting such custody. This process can be written by the court in any way the court believes will facilitate decision making by the custodians and resolve any disputes.
One way courts often resolve such disputes is by mandating that in the event of a serious dispute the parties must attend mediation with the goal of resolving the dispute in good faith and, if that process should fail, the parents are then to petition the court for a resolution of the issue.
Another way is to designate one of the custodians as a final decision maker, that is, if the parties disagree then the final decision maker must consult with the other custodian to resolve the dispute, and notify them of their decision after the consultation. After this process, if the dissenting parent still disagrees, they may petition the court.
Yet another way is to designate one custodian as the final decision maker on certain issues, and the other custodian as the final decision maker on other issues.
The courts understand that there will be disputes regarding important decisions affecting a child's future, but in entering a Joint Custody Plan (which requires the agreement of both parents) it has already been determined that the parents, despite their personal differences, are likely capable of successfully co-parenting their child. I believe that most of the time this is true and that Joint Custody Plans serve the best interest of children.
Occasionally, however, one party or the other files a Motion to Terminate Joint Custody which asks the court to end the Joint Custody Plan and establish sole custody in one or the other parent. A typically bitter court fight follows which further cripples the parent's ability to successfully rear their child, but does result in clarity regarding how decisions are made from then on. The winning parent decides.