How can hydrogen and oxygen coexist in high temperature? Where did the oxygen come from?
I'll answer the question in general of how Hydrogen and Oxygen can coexist in metastable mixtures at higher temperatures, though without hazarding a guess for your specific case. Like you and others, I'd be very surprised if your bulk gas is actually well mixed at $\pu{900 ^{\circ}C}$ without exploding! It's far enough beyond the autoignition temperature, that an explosive reaction seems inevitable. However, metastable combinations of methane, hydrogen and oxygen in the $\pu{550 ^{\circ}C}$ range have been reported in the literature.

Hydrogen can be a surprisingly picky burner. I was told by a colleague who specialized in rocket propulsion that in between Hydrogen's upper and lower explosive limits are a host of regions where it strongly resists ignition, even above its reported autoignition temperature of ~$\pu{570 ^{\circ}C}$. According to her, the reaction between hydrogen and oxygen is quite complex with several important intermediates. Having concentrations in these non-explosive regions apparently allows the hydrogen-oxygen mix to be metastable for long periods. This would of course not extend to long time periods at $\pu{900 ^{\circ}C}$!

More in my territory, there's also many chemicals which inhibit the slow combustion (non-explosive) of hydrogen and oxygen up through the $\pu{500 ^{\circ}C}$s. The most common is $\ce{KCl}$, which strongly inhibits the reaction of oxygen and hydrogen in non-explosive conditions allowing a metastable mixture to exist. Walls of vessels can also similarly inhibit reaction. The reaction mechanism apparently stems from $\ce{KCl}$ and other surfaces' abilities to consume reaction intermediates of the combustion such as monatomic Hydrogen:

$$\ce{KCl + H^. -&gt; K^. + HCl}$$
$$\ce{SiO2 + H^. -&gt; SiO(OH)}$$
or direct adsorption on metal surfaces like nickel, palladium, etc.

After doing a bit of digging, methane is apparently a superb inhibitor of hydrogen and oxygen combustion. The effect is explained in a 1959 paper by R. Baldwin, et al. (see following reference). The paper explains that when methane is above a sharp critical concentration it drastically raises the autoignition temperature of hydrogen's explosive reaction and almost completely eliminates slow combustion in non-explosive conditions. In other words, when enough methane is present, the hydrogen and oxygen mixture either does nothing or explodes completely. From data taken in systems between $\pu{450\! -\! 550 ^{\circ}C}$, there was a very clear relationship of methane concentration vs. explosive limit taking the form of (values approximate):

Explosive Limit (in $\pu{K}$) = $\frac{\pu{3350K}}{2.3-\log_{10}(X_\ce{CH4})}$

where $X_\ce{CH4}$ is the mole fraction of methane, studied in the 0.01 - 0.1 range (significantly lower than yours). For this particular heuristic, the mole fraction of hydrogen was 0.28, oxygen 0.14, and the balance was nitrogen. The paper suggests the mechanism is non-equilibrium formaldehyde formation.

As a final note, please be very cautious when handling or recreating these mixtures. Just because the hydrogen isn't burning, doesn't mean it can't explode. A lab down the hall from mine was fortunate to loose only a wall to a hydrogen evolving system which ran fine for months and then, quite suddenly, decided to explode.

Best of luck learning more about your system! It sounds fascinating.



Reference:

R. R. Baldwin, N. S. Corney, R. W. Walker, “The inhibition of the hydrogen + oxygen reaction by methane,” Trans. Faraday Soc. 1960, 56, 802–814 (DOI:10.1039/TF9605600802).