Hide and Seek

Ollie ollie oxen, all in free.
No one has discovered me.
I’m sealed up in my hiding place
getting ready, just in case.
I am not easy to figure out,
but if you do, without a doubt,
I’ll be right here in the middle,
once that you have solved the riddle.
But ollie ollie oxen, all in free.
First you have to come find me.

The NaPoWrMo prompt today was “middle.”