Butt, Buttocks, Bottom
noun
The お is just there to be nice to your butt. This word simply means butt (or any other word for it, like buttocks or bottom).
おしり
The reading is the same as the one you learned with the kanji.
お尻にイボができました。
I got a wart on my buttocks.
高いところに行くと、お尻がかゆくなるんです。
When I go to high places, my butt itches.
これはお尻のインナーマッスルのエクササイズです。
This is an exercise for the inner muscles of the buttocks.