An opportunity attack uses your reaction:Sentinel is based on opportunity attacks and says nothing about "extra reaction attacks", and Riposte does not say the riposte attack is an opportunity attack.
Is an extra reaction attack the same thing as an opportunity attack?
Riposte is another way to get an attack with your reaction. There can be many situations where the mere threat of a Sentinel-based opportunity attack will cause the enemy to not move away, and so having other ways of getting those extra reaction attacks is good, which is why Riposte is so valuable.You can make an opportunity attack when a hostile creature that you can see moves out of your reach. To make the opportunity attack, you use your reaction to make one melee attack against the provoking creature. The attack occurs right before the creature leaves your reach.