A method holding an auction for impressions in an online advertising platform includes defining auction tiers, with each tier having associated attributes. The tier attributes include a tier priority, an eligible bidder list, and a minimum bid threshold. Bids for the impressions are received from impression buyers, each bid being associated with an impression bidder and a bid value. For each received bid, the highest priority tier for which the tier attributes are satisfied by the bid attributes is determined, and that tier is associated with the bid. After assigning each bid to a tier, the highest priority tier with at least one bid is selected as the active auction tier. The winner of the auction is then determined based on the bids associated with the active tier.