Chill. just tell yourself this planet has strong abiotic oxygen production characteristics or that it's in an ice age with a highly oxygenated ocean beneath the ice acting as an oxygen reservoir for the planet. aeroplankton floating in narrow warm bands in the middle atmosphere? if we're already operating in a universe where The Force and Lightsabers exist, then I am begging you to please take the easy out when facing questions like "what are the realistic atmospheric compositions of an ice planet". whatever you told yourself to sleep at night after seeing Hoth, that is your new mantra