What is the EXACT diameter of your bore? Not all 20 gauge barrels are the same.
My "20gauge" barrel turned out to measure .626-.627", not the normal .615". My bore is much closer to 19 gauge( .629") than to 20 gauge, and I have bought 19 ga. OS cards to use in it instead of the 20 gauge cards. The 20 ga. OP wads, and cushion wads work, and are "easier" to load because of the larger diameter bore. But, being thicker, and softer, they compress and fill the bore nicely. The OS card has to be bigger because its thin, and has to hold back the shot.
It also helps to use TWO OS cards-one on top of the other-- for these heavy loads of large shot pellets. 90 grains by volume of #5 shot is 1 3/8 oz. of shot by weight, nominally. That's a huge load for a 20 gauge- and more fitting to use in a 12 gauge.
For a better pattern, back that load of shot to 1 1/8 oz., or 75 grains by volume.
An equal load of 70 grains( 2 1/2 drams) of powder would be 1 oz. of shot by volume. That is considered a heavy load for 20 gauge guns.
You can back that powder charge back to 55 grains( 2 drams) and shoot 7/8 oz. of shot( 68 grains by volume) and most guns will give good patterns.
You can buy 19 gauge cards from many suppliers, or from Circle Fly directly.
ALWAYS measure the diameter of your bore- land to land in rifles-- when you take possession. I learned that lesson the hard way. It just is not always the bore( caliber) ga./diameter marked by the manufacturer on the barrel. It happens more than you can ever know.