Far Back
In The Back
independent noun
Something that is in the interior of something else is inside of it, deep, and far back / in the back. Just imagine something interior to something else, it's probably in the back.
おく
The reading is the same as the one you learned with the kanji.
奥にもっと小さいサイズのものも置いてますよ。
We have ones in smaller sizes in the back.