The mag limit is for .22 centerfire rifles only, during OK deer gun season while deer hunting.
Any other time there is no limit on what you can carry in the field.
Legal Means of Taking
Rifles: Centerfire rifles firing at least a 55-grain weight soft-nosed or hollow-point bullet and having an overall cartridge case length of 1 1/4 inches or longer (9mm rifles are not legal). Clips or magazines of all .22 caliber centerfire firearms may not be capable of holding more than seven rounds of ammunition.
http://www.eregulations.com/oklahoma/hunting/deer-season/
I've heard conflicting stories about how that came about. The one that seems to hold the most plausible explanation is that a legislator, put that in a bill as "common sense " measure because the 30-30 lever gun only holds 7.
However, I've been told that no law gets passed about wildlife without going through the wildlife commissioners.
Never been able to get any reasonable explanation why only .22 centerfire from anybody.
Meanwhile there are no restrictions on mag capacity for calibers other than .22 centerfire during deer gun season while hunting.
I have need advised by a couple of game wardens to let sleeping dogs lay, as too much protest may inspire those that can/do pass laws to put mag restrictions on other calibers.