When I was fitted at @EuropeanSleep for my O2 pillow, I ended up with the firmest and tallest option they had, possibly even firmer and taller than what is currently listed. I also place it on top of a base king pillow, which in my case is a Tempur-Pedic Cloud and Conforming Pillow from their 2017 or 2018 collection. I am 6’0", about 220 lbs, with broader shoulders, and I strongly prefer a firmer pillow overall.
Lately, I have developed a bit of a routine with my pillow setup. The Tempur-Pedic base stays constant, but I start the night sleeping on my horsehair-filled pillow as it is very firm but comfortable. In the middle of the night, I usually swap that out for the O2 pillow. The O2 has a slight bounce to it, but not nearly as much as a typical latex pillow, which I appreciate since too much bounce can be distracting.
When I get up briefly during the night and return to bed, I switch to the O2 pillow and position my head near the corner. That gives me the best combination of support and a softer feel along the side of my head.
If you are built like I am, having a couple of pillows to rotate can be really helpful. As for the O2 specifically, I strongly prefer it in the firmest and tallest configuration available.
Maverick