Opportunity attacks are only when an enemy leaves your reach
You can make an opportunity attack when a hostile creature that you can see moves out of your reach.
The exception to this is if you have the feat polearm master:
While you are wielding a glaive, halberd, pike, or quarterstaff, other creatures provoke an opportunity attack from you when they enter the reach you have with that weapon.
A shove will never provoke an opportunity attack regardless
Even polearm master won't let you take an opportunity attack against the shoved enemy since in order to qualify for an opportunity attack the enemy has to move using their action, reaction, or movement:
You also don’t provoke an opportunity attack when you Teleport or when someone or something moves you without using your Movement, action, or reaction.
A shoved enemy is moved by your action and without using any of their actions or movement. So it will not provoke an opportunity attack.