This is an issue I've seen coming up with the addition of the Eldritch Claw Tattoo in Tasha's Cauldron of Everything, though there seem like there were possible cases of it before.
As far as I can tell, every character starts with a reach of 5 feet. If they are wielding a weapon with the Reach property this explicitly increases the reach of both their attacks, and their reach for the purposes of being able to perform an opportunity attack reaction, by five feet:
This weapon adds 5 feet to your reach when you attack with it, as well as when determining your reach for opportunity attacks with it.
However, an Eldritch Claw Tattoo's Eldritch Maul effect doesn't mention opportunity attacks in the same way, it only mentions the range of your attacks:
As a bonus action, you can empower the tattoo for 1 minute. For the duration, each of your melee attacks with a weapon or an unarmed strike can reach a target up to 15 feet away from you, as inky tendrils launch toward the target.
There is a similar case for the playable Bugbear race from Volo's Guide to Monsters, which has the long-limbed trait:
Long-Limbed. When you make a melee attack on your turn, your reach for it is 5 feet greater than normal.
In both cases it sounds like the additional reach only applies when you actually attack; it's unclear whether your reach is also increased for triggering an attack of opportunity or not.
I think the tricky point is that the reach that determines whether you can make an opportunity attack, occurs before you're actually making any kind of attack roll (i.e- it determines whether you can use your reaction or not), whereas the latter two quotes suggest your reach is only increased when actually attacking (about to make the attack roll). As far as I can tell (though I may have missed it) the rules don't state that your reach is based on your melee attack reach, it just happens to be convention that some effects that increase one, also increase the other.
This feels like it might be a bit of a weak distinction, but it also feels very strange that a temporary increase in range should render a character unwilling to strike at an enemy that they would have attacked just a moment before.
So my question is, is there anything in RAW that makes clear that Eldritch Maul, Long-limbed and so-on should also apply to reach for the purposes of triggering an opportunity attack, or am I correct in thinking these may be separate?