You don't get your sneak attack dice. Combat Advantage occurs either at the end or after the attack resolves.
When you make an attack with a light blade, a hand crossbow, a shortbow, or a sling and hit an enemy granting combat advantage to you, that enemy takes extra damage based on your level (see the Sneak Attack table). You can deal this extra damage only once per turn."
Sneak Attack is triggered/happens when you make an attack against someone already granting you combat advantage. I'm not 100 percent sure if Opportunity Knocks applies the combat advantage on hit (but before damage dice) or after the attack resolves, but sneak attack seems to require the combat advantage prior to the attack roll being made.