Quote from MTE:
It can be either one, depending on what the seller is doing (i.e. selling to open or selling to close). If the seller is selling to open then a new contract is created, if selling to close then no additional contract is created.
Quote from nazzdack:
FWIW..............If a new "long" trades with a new "short", open interest increases. If an open "long" trades with an open "short", open interest decreases. If a new position holder trades with an open position holder, open interest remains unchanged.