You cannot select occupied Monster Card Zones with “Ojama King”’s effect. If your opponent has 5 monsters on the field, then “Ojama King”’s effect is not applied.
“Ojama King”’s effect activates and you select up to 3 Monster Card Zones when “Ojama King” becomes face-up on the field.
If you select up to 3 Monster Card Zones and your “Ojama King” is flipped face-down, the Monster Card Zones are usable again. When “Ojama King” is flipped face-up again, you can select up to 3 Monster Card Zones again.
If you take control of your opponent’s face-up “Ojama King”, you cannot select new Monster Card Zones because the zones are selected when “Ojama King” becomes face-up on the field.