So I’ve been delving into the subject of opportunity attacks

it seems they are reaction, one of these per turn, for enemy leaving range of weapon.

i see a couple feats that increase the range and or allow OA when enemy enters or leaves range.

is there any combination of RAW which allows for more than one OA or grants someone else an OA?  

Im still rereading all the classes and spells and specialization seems, just a lot to absorb

No RAW grants more than one OA, it's an intentional limit built into the game.   There are some things that let you grant someone ELSE the right to take a Reaction on your turn.  ( Battlemaster's Commander's strike).  Not exactly an OA, but close.

If you have War Caster you can use your OA to cast a spell - such as Magic Missile (multiple missiles, 1 spell)  or Eldritch Blast  (multiple attack rolls once you hit 5th level).  Again, close to multiple OA, but not exactly.

Fighter (Cavalier) gets Vigilant Defender at 18, which gives you a "special reaction" that lets you make an OA on every turn except 1) the turn you use your regular Reaction and 2) your own turn. That's the only exception I know of.

Also, just remembered, certain high powered creatures get legendary actions, which are similar to reactions.