Could be a mic pack, however:

1.) It’s deep in her boot. Too deep for it to be a practical location for a mic pack. It looks like a knee high boot and would be better sitting closer to the top hole of the boot. She does have a mic on. It’s on her right side lower collar where the white under shirt meets the pink sweater.

2.) IF it’s a mic pack, it would have to be wireless, because you can’t change sitting positions and certainly can’t walk with a wired mic pack without pulling on the wire.

3.) With the way she is positioned and the size and shape of that chair she is on there are better places to stick a mic pack on or around the chair or on Oprah herself. Both Harry and Megan have little mics clipped on their tops and you don’t see their mic packs protruding anywhere on either of them.