Actually, cooling a dry fuel would be the reason that we put water on it when it's on fire.
And water is 1400+ times a better cooler than air ever could be except when it too is a liquid.
That cold air doesn't stand a chance of cooling that pellet fire enough to make a difference and in cold weather the fire will definitely have a better oxygen supply.
You need to look for any restriction in that system, an OAK is just another vent system.
This could be something as little is a glob of RTV that blocks part of the cross sectional area of the OAK or even a slightly crushed section of the pipe.
What end did you disconnect from the system?