I've been putting my Oxygen through its paces for a few days now. I bought a used set of the black variant in like-new condition. My tl;dr is that the O2 is extremely good, in fact I'd say it's mostly great. It has better technical abilities than most of the IEMs in my collection, the only nitpick is the tuning isn't quite aligned with my ideal preferences - the upper mids are a bit too prominent/intense.
The fit doesn't bother me in regards to comfort, but it required a lot of tip rolling to find something that will keep a stable seal with the short nozzle. I had success with Symbio W hybrids, but have settled in with KZ Starlines. Build quality on the actual shells is excellent overall, the cables seem fine, if a bit thin. I think I want to experiment with some other cables, and I'll need to do some research on suitable options.
Bass is very nicely done - good sub bass extension, prominent and "musical" mid bass that doesn't color the midrange at all. It delivers clean hits that have good punch and clarity, but linger long enough so that things never sound sterile. O2 doesn't have the skull rattling power of some other IEMs I own (Ikko OH10 and TFZ No. 3 come to mind), but it never feels lacking for extension or impact to me, and is less distracting/fatiguing than the "basshead" sets. Drums and electric bass in rock music sound great; the Oxygen is well suited for the fast paced kick drums and bass in metal music.
Upper mids are definitely more prominent than lower mids, there isn't much in the way of warmth or lushness in the mids. Certain electric guitar, keyboards or upper register vocals can get pretty intense, which is a complaint I have about many other IEMs I own but otherwise still enjoy (OH10, No. 3, Kanas Pro being good examples). I will note that the O2 never feels "shouty" to me. Vocals are presented in a somewhat lean but clear and natural fashion. Imaging and layering/separation are superb, probably the best in my collection, which really helps bring the mids to life. My ideal preference would have better balance between the lower mids and upper mids, but the overall presentation is very well done.
Treble is well extended with good sparkle, never harsh, and adds a nice amount of airiness to the overall experience. Things like cymbals and high hats are very prominent, and are always presented with clarity and good placement in the overall stage. The overall tuning here reminds me a lot of the Kanas Pro, but the O2 sounds less colored, and the O2 is in another league when it comes to crispness, detail retrieval and clarity. O2 can be unforgiving on poorly recorded/mixed source material.
Technicalities on the whole are excellent. Sound stage has natural-but-not-exaggerated expansion in all dimensions, with surprisingly good height and depth representation. While it's not necessarily "outside your head" expansion, the well rounded presentation is fairly unique, at least compared to the rest of my collection. Separation and layering give each instrument/performer their own location. Listening to well recorded live albums, you can place the performers in their arrangement on the stage. The combination of the sound stage and the layering/imaging frequently provides the oft desired "3D/holographic" effect that is difficult to come by in IEMs. Spatial cues are rendered better on the O2 than anything else I own.
Comparisons
Before I acquired the Oxygen, I had 4 IEMs in my collection that to me were in another league than the rest of my collection, JVC FDX1, Ety ER2XR/ER4XR, and Yamaha EPH-200, with the Ikko OH10 just a bit behind those. The Oxygen safely fits into this top tier of my collection, and may be the best of the bunch due to it's unique sound stage and imaging.
The set that first came to mind when I initially started listening to the Oxygen was the OH10. Both have an upper mid bias, sub bass extension, and pleasing imaging and stage. The OH10 has more bass impact and slam, the Oxygen has better control, texture and "musical" presentation of bass. Upper mids are even more pronounced on the OH10, which makes it veer into being "shouty". Both have good treble extension, with the Oxygen feeling a bit more crisp and precise. The Oxygen is superior when it comes to sound stage depth/height and separation, which is notable because OH10 honestly aren't bad in those areas.
The Oxygen also shares similarities with the Yamaha EPH-200 to me. They both do well in imaging and (especially) separation. EPH-200 has slightly bigger and bloomier bass, Oxygen bass has better control and texture. EPH-200 is a bit more forgiving overall, whereas the Oxygen has more treble sparkle and detail retrieval. The upper mid elevation feels less extreme on the EPH-200. The fit of the EPH-200 doesn't bother me for the most part, but the Oxygen is more comfortable, and is much better suited for tip and cable rolling. The Oxygen does have the edge on stage expansion, even though the Yamahas fare well there too.
ER2XR has less intense upper mids and less treble extension/sparkle than the Oxygen. ER2XR offers good detail retrieval while still being fairly smooth, where the Oxygen can be more unforgiving. I think the ER2XR is able to provide slightly more clarity and detail with vocals, but Oxygen's separation can actually give vocals more room to shine when dealing with busy tracks/arrangements. Stage, imaging and separation are vastly superior on the O2; ER2XR presentation feels very narrow/shallow and compressed by comparison. ER2XR has a hint of bleed from the bass into the mids, so it's less clean but a bit warmer than the Oxygen. Male vocals on the ER2XR have more body. I think the Oxygen has better and cleaner presentation of bass on bass-heavy genres/tracks, but both offer a nice musical bass representation. Overall, when A/B listening to them back to back, ER2XR does feel a bit dull-but-safe by comparison, in large part due to the Oxygen's superior imaging, treble sparkle, and stage.
I use the JVC FDX1 exclusively with the nozzle that does the most dampening. Bass has a bit more prominence and impact on the Oxygen. Both have a bit of a "lean" approach overall, with the FDX1 being a bit leaner/cooler. With my preferred nozzle filter the FDX1 seems to have better balance between lower mids to upper mids, which is more enjoyable and natural sounding to me. Oxygen has better stage depth and height. Overall, the FDX1 has a frequency response I prefer, and is a bit smoother, but the Oxygen has better imaging, separation, stage depth/height, and more satisfying bass impact. FDX1 is more refined, Oxygen is more exciting.
Summary
I am very happy that I've added the Oxygen to my collection. It's extremely well built with an attractive design, and the nozzle length and shell design thankfully haven't posed comfort or seal issues for me. It's imaging and sound stage capabilities are the standout features. Lower mids and male vocals are a bit thin, but are still very clean and well represented, thanks to the set's technical abilities. Upper mids and treble have a lot of detail and emphasis, which can make them a bit unforgiving or intense depending on your preferences. Bass has good extension that slightly favors sub bass to mid bass, with a clean yet textured presentation.
If forced to choose, I'd probably waffle as to whether the Oxygen or FDX1 is the best set I own, depending on source material or my mood. The Oxygen might not be a good choice if you're prioritizing smoothness, lushness, or warmth.
