These look like truly amazing IEMs! I'm curious though, would you consider the midrange to be linear? How would you compare them to the Tin T2 or the HD 650 (assuming you've heard both)? And is the lower treble an issue at higher volumes?

Midrange feels to be fairly linear but the entire picture would be closer to very light U-shape. Very small elevetion of lows (to compensate human lower perception of bass) and a bit more elevation on treble. But the mids sound quite even when it comes to female|male vocals
No, for me (subjectively) - Oxygen IEMs are not having this peak in lower treble that would be piercing at higher volumes. I own many other IEMs that would sound harsh there. Not this case.
Cheers

How would compare these to Kansas or Kansas Pro or say even the DM6's? have you tried these balanced out of your HiBY R6 Pro ?

Thanks

I also own the Oxygen and agree totally with your excellent review! The detail, imaging and overall sense of space is amazing, especially so at this price point. IMO, it can compete favorably with the latest boutique "flagships"!