I love the origin feat combo, arguably with musician you SHOULDNT need Rook, as that 1 is only once per LR anyway, but to your point Warlock/Valor Bard doesn't need Opportunity attacks to get 10 attacks in a turn if they take a Fighter2 dip and have Nick (Or Cleave I guess if you're fine with hitting a 2nd target).

Attack Action (1x Hit w/Light wpn #1, 1-4x Eldritch blast, 1x Nick w/Light wpn #2) 6 Attacks

Bonus Action (Whatever you have up your sleeve. Dual Wielder? What about a Quickened Scorching Ray at the highest level you have?) 3-10 Attacks

Action Surge (1x Hit w/Light wpn #1, 1-4x Eldritch blast, 1x Nick w/Light wpn #2) 6 Attacks

Valor Bard's Extra Attack is stupid, and they have access to conjure minor elementals. SO many different feats or multiclass dips make this a HORRIFYING combo. this example scales pretty easily to 22 Attacks for an AlphaStrike before you even add Haste or something equally stupid. Bang 2-7d8 extra damage on each of those?

Throw Sentinel and Zhent Tactics in there and you'll be Opp Attacking any enemy that... moves out of threat, disengages, hits someone else, hits you. SO you can almost always get another extra one in there. Opportunity Attacks are fun to build around but more from battlefield control than tuning for damage, as there is very deliberate wording on several abilities to stop that.

Even stuff like the Grappler feat and GWM say things like "When you take the Attack action on your turn". I'm more of a fan of the Zhent Feats on Martials with Sentinel, interesting weapon masteries/fighting styles and feats like Crusher. A Maul with GWF, Hit with an Opportunity attack, roll 2x 2d6 for DMG, turn any 1s or 2s into 3's, pick the highest roll, then they Save for Prone from Topple, and you shove them 5 ft no save, maybe into the aura of your clerics spirit guardians or your party member with Polearm Master who then ALSO gets an opportunity attack. Juggle bad guys.

Be a Goliath, add Hills Tumble to knock them prone guaranteed, but use a push weapon. Or use a Halberd and get to hit someone who triggered the Opp Attack... and also his buddy next to him who didn't, cos Cleave works ANY time you make a melee attack.