A Put is at the money (ATM) when the strike price and the stock price are the same .Even if the stock price is near and not exactly as the strike price it is considered ATM.
Xpert Comments
ATM puts are bought by traders who expect the stock to decline and instead of short selling futures and increasing the risk factor they buy puts.This happens on a higher side when there is a profit booking , an intraday reversal or political/global uncertainities ,there is a tremendous Put OI increase in ATM puts.