I don't mean to come off as rude, but if "Big O Rock" is your only way to describe what your picture doesn't tell us, then "Big O Rock" is about all you'll get back. Rocks are hard to ID, and nearly impossible to ID by picture alone.
Take off a chip at a small spot on it and show us a picture of the inside, that'll tell more. When rocks get weathered like that on the outside we call that a cortex. My wild out of the blue guess is that it's a misshapen flint nodule, small hammer chip off the side will probably confirm.
I'm new to this forum.I'm still tryin to figure out how to use this site on my cell phone there's more room for improvement.and I don't take it as being rude I take it as advice.Thank u icewing