Genesis is understood as not exactly mythical but certainly allegorical by most mainstream Christian denominations, including Catholics, Angloians, Methodists and Presbyterians.
Other planets do not have free O2 in the atmosphere because O2 is reactive and would disappear over time. On the Earth, it is continually renewed by the photosynthesis in plants. When life first started there was no free O2. The first organisms were anaerobic, as many bacteria are to this day. When photosynthesis evolved, initially in cyanobacteria, free O2 began to accumulate, This event is marked in the geological record by bands of oxidised minerals and is thereby dated to about 2 billion years ago.