Learn German: Wir warten, bis das Brot im Ofen aufgeht. - We wait until the bread rises in the oven.

The word bis (until) is a subordinating conjunction. This means it introduces a dependent clause and forces the conjugated verb to the very end of that clause. So instead of a standard word order, the verb is pushed to the final position.

Isn't aufgehen a separable verb? Why is it written as one word here?

Yes, aufgehen (to rise) is a separable verb. In a regular main clause, you would split it: Das Brot geht im Ofen auf. However, because the conjunction bis pushes the conjugated verb (geht) to the very end of the sentence, it bumps right back into its prefix (auf). When they meet at the end of a subordinate clause like this, they are glued back together into a single word: aufgeht.

Why does the sentence use im Ofen instead of in den Ofen?

The preposition in is a two-way preposition. It takes the accusative case when showing movement into a new place, and the dative case when showing a fixed location. Since the bread is already resting inside the oven while it rises, it is in a fixed location. We use the dative case dem Ofen, which combines with in to make im.

Can I use aufgehen to say that I am getting out of bed?

No, you wouldn't use aufgehen for a person waking up or getting out of bed (that would be aufstehen). aufgehen is specifically used for things like dough or bread rising, a flower opening up, or the sun rising.