The cleric’s Trickery Domain subclass has the Channel Divinity feature “Invoke Duplicity” which creates an illusory duplicate of the cleric and provides the following abilities:
For the duration, you can cast spells as though you were in the illusion’s space, but you must use your own senses. Additionally, when both you and your illusion are within 5 feet of a creature that can see the illusion, you have advantage on attack rolls against that creature, given how distracting the illusion is to the target. (PHB, pg. 63)
Can this duplicate make opportunity attacks? Or, more precisely, can the cleric make opportunity attacks through their duplicate?
If the cleric has the War Caster feat, they are able to use certain spells as opportunity attack “replacements”:
When a hostile creature’s movement provokes an opportunity attack from you, you can use your reaction to cast a spell at the creature, rather than making an opportunity attack. The spell must have a casting time of 1 action and must target only that creature. (PHB, pg. 170)
This would be necessary for allowing opportunity attacks via the duplicate because the cleric can only cast spells through them, not make weapon attacks there. (Spells with 1-action or 1-bonus-action casting times cannot be used as opportunity attacks, as discussed by Sage Advice here.)
So I think this comes down to a question of reach. Does the illusory duplicate extend the cleric’s reach to include the 5 feet around the duplicate because the duplicate allows the cleric to cast spells “as though (they) were in the illusion’s space”? They are allowed to make melee spell attacks within 5 feet of the duplicate, after all. Or is the cleric’s reach limited to the 5 feet around them, personally?