What Is Aggressor Bias?

Every trade has a buyer and a seller — but one of them was usually more eager to get the trade done. Aggressor bias is the attempt to figure out which.

In any options trade, there's a resting bid (what buyers are offering to pay) and a resting ask (what sellers want to receive). When a trade actually executes, its price relative to that bid-ask spread gives a clue about which side was more willing to move to get filled — this is called the trade's aggressor, or sometimes the "aggressor side."

Why it's called a "bias," not a fact

Aggressor classification is a probability, not a certainty. Prices move constantly, and a trade filling near the ask a few seconds after the ask itself moved can look misleading if the reference price used is even slightly out of date. It's called a bias because it's the more likely read given the available information — not a confirmed fact about who initiated the trade.

Aggressor side alone doesn't tell you direction

A buy at the ask on a call option is typically read as bullish. But a buy at the ask on a put option usually means the opposite — the trader is paying up to bet on a decline, or to hedge. Reading aggressor bias correctly always requires combining it with which type of option (call or put) was traded, not just which side was more eager.

Why it matters combined with other signals

On its own, aggressor bias is a soft signal — useful, but imperfect. It becomes more useful when combined with trade size (was this a meaningful trade?) and OI confirmation (did it actually build a new position?). A large, ask-side, OI-confirmed call buy is a materially stronger combined signal than any one of those three facts alone.