Who is the very first buyer of a futures contract? Don't tell me the first seller of course because then it's spontaneous to ask who did that seller bought from in the first place ?
Open interest represents the number of contracts that exist on any given day. There are 4 scenarios:
(1) If both parties to a trade are initiating a new position (one new buyer and one new seller) then open interest will increase by one contract.
(2) If both traders are closing an existing or old position (one previous buyer and one previous seller) then open interest will decline by one contract.
(3) If someone long an option sells to a new trader, open interest will not change (contract changing hands)
(4) If someone short an option buys from a new trader, open interest will not change (contract changing hands)
(1) applies to your question. There are no contracts in existence so a willing buyer and a willing seller agree on price and a contract is created.
"Buyer of a futures contract" is a bit misleading, although I realize this is the standard phrasing. You are not actually "buying" a contract - you are entering into a contract in which you are the buyer of a commodity. The other party in that contract is the futures exchange which also creates a contract between them and the "seller".
If you are buying to close out an existing short position, the exchange nullifies your original contract rather than creating a new contract that offsets the original one.
Whether the "seller" is opening or closing a position is of no consequence to you. The exchange handles all of this and keeps track of "open" contracts (as Bob's answer illustrates).