I am interested in the OP-Amps for the Fosi Audio V3. How do I differentiate the various options?

Different op-amps can produce varying sound qualities.

You can refer to this video showing different dual op-amps for comparison: https://www.youtube.com/watch?v=nrqVYE4_v1M

Here are some genuine suggestions from users.

Ranking: Sparkos SS3602, OPA2604 and... JRC4580D, NE5532 small differences MUSE02 the worst! Weird sound, where are the bass? Sparkos is too expensive, then OPA2604 is the best choice. To the future, maybe OPA2828 with DIP adapter

NE5532 vs OPA2604 -> the second is more musical, better voice

Texas Instruments (opa2604) for me ! these opamps rock / clear dynamic and incredible 3d soundstage !

I confirmed that the LME49720 is also a great drop-in replacement. It opened the sound stage and gave me a tighter low-end. I was truly impressed!

I messed about with various op amps including Oracle 02, Muses 8920d, various 5532s and eventually settled on the Burr Brown opa2604 which has the best soundstage out of all them, very wide and well detailed with a nice overall balance and good dynamics. I will try the opa1622 when they arrive as they sound similar to the opa2604 in other amps I have tried but adding a little extra air in the upper frequencies.

To further add, my V3 with the opa2604 sounds almost identical to my Onkyo A-9010, I like it!

I'm testing the Fosi AUdio V3 with the new ORACLE II-02 Op amps, the result is really exciting, the result is a sound no better than that produced by the NE5532s, but certainly different, warmer , much more similar to traditional amplifiers.