Can a Defender's interrupt ability stop an opportunity attack? If so, can I get a page reference?
Yes, the defender's action will interrupt -- and under certain conditions it will render ineffective-- the opportunity attack.
They are both essentially immediate interrupts. The opportunity attack is triggered by an external event, the defender's feature is triggered by the OA. The OA interrupts the external trigger, but the defender's feature interrupts the OA.
This means that if the defender's feature causes an effect which makes the OA impossible, then the OA is wasted.
That was confusing.
Let's dig into the rules, shall we?
An opportunity action is not an immediate interrupt, but it acts like one:
Now, an opportunity action is not always an opportunity attack.
Barring exceptions, the opportunity attack is triggered by movement or by a ranged attack. Let's say it's a ranged attack, for the purposes of example [largely because if our OA is provoked by movement, then we add the extra layer of resolving the move, and that's not necessary to answer this question].
Heroic ranger Dae'eravin shoots an arrow while villain Davith the Wicked is standing next to him, provoking an opportunity attack. Davith takes the opportunity attack, but Dae'eravin's fighter friend Kamola is standing right there and has Davith marked.
Kamola decides to use his Combat Challenge power in response:
The Combat Challenge attack interrupts Davith's opportunity attack, and if it hits then Davith will take damage before he can roll to attack Dae'eravin. If this damage drops Davith to 0 or fewer hp, then Dae'eravin is never hit because the Combat Challenge damage landed before the opportunity attack could be made.
If the damage from Combat Challenge does not incapacitate Davith, then Davith still gets to make his opportunity attack after Kamola has smacked him upside the head.
You can improve your chances of stopping actions with your interrupts
What if Kamola had the Shield Bash feat?
Now even if he doesn't drop Davith from damage, on a hit he can still push Davith away from Dae'eravin and make the opportunity attack fail because Davith can't reach the ranger to stab him.
There are a number of feats, items, and features, which improve defender interrupts in this way. Most, like Shield Bash, are tailored to a specific class's powers and features.