You should almost never discard the okey, because it is the most powerful tile and finishing on it scores double.

The joker completes any run or set, so holding it keeps your options open until the last moment. Only let it go in rare cases, such as when you already have a locked-in finish that does not need it. Even then, think twice: handing the okey to a rival can instantly complete their hand. In most situations, keeping the joker and discarding a safer tile is the stronger play.