There is a gray area for characters armed with two weapons having different reaches. IMO it makes sense to allow an OA with a 5 ft reach weapon when an opponent moves from 5 ft to 10 ft away, even if you are carrying a 10 ft reach weapon as well. And since everyone has an unarmed strike, that option is basically always available.
To me that just seems logical, and I think it works fine in the game because it gives you a choice to exert more precise battlefield control at the cost of (typically) lower damage on the OA.
Of course, this doesn't let anyone make two OAs, since you still just have one reaction.
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Oh and I see the sage supports this interpretation:
https://www.sageadvice.eu/2016/10/0...reaches-when-does-it-get-opportunity-attacks/
Still, I wouldn't argue much against a DM ruling the other way.