In our game we have recently found a Tentacle Rod/Rod of Tentacles found in the DMG.
The weapon states
While holding the rod, you can use an action to direct each tentacle to attack a creature you can see within 15 feet of you.
Because it requires the PC to use an action to direct the weapon to make an attack does that mean you would not be able to make Opportunity Attacks as they require a Reaction not an Action?
The "Use an Object" section of the PHB states:
When an object requires your action for its use, you take the Use an Object action.1
Thus, activating the Tentacle Rod requires your action and opportunity attacks do not allow you to take actions, only reactions.
This is similar to the fact that you cannot grapple/shove as an opportunity attack because they require the Attack action, which you cannot make with an opportunity attack. The Sage Advice Compendium explains this as follows:
Q. Can an opportunity attack be used to make a grapple or a shove?
A. Grappling and shoving are special melee attacks that require the Attack action (PH, 195). An opportunity attack is a special reaction. Take the Ready action if you want to attempt a grapple or a shove as a reaction.
1 The DMG (page 141) clarifies this further:
If an item requires an action to activate, that action isn't a function of the Use an Object action, so a feature such as the rogue's Fast Hands can't be used to activate the item.
Using the Tentacle Rod does still require your action, but it specifically is not the Use An Object action. This would be the Activate An Item action or perhaps the Tentacle Rod action.
Your reasoning is correct. Activating the Tentacle Rod isn't an attack, so it can't be an opportunity attack. (The Rod itself makes attacks, but only when you activate it--it doesn't get its own turns, so it can't take reactions of its own.)
If you really want to use it reactively, you can ready an action to activate it, but that's at the cost of doing anything else with your action on that turn.
