Abstract: An online system communicates information describing deals guaranteeing impressions of content items within specified time periods to a content publisher at which the content items may be presented. The online system receives information describing an assignment of each deal to a placement position within a website for the content publisher. Upon receiving a request to present a content item to a viewing user of the online system at a placement position, the online system identifies candidate content items associated with deals assigned to the placement position, in which the candidate content items are associated with targeting criteria satisfied by the viewing user. Based at least in part on a number of impressions achieved by each candidate content item in relation to a number of impressions guaranteed for the candidate content item, the online system selects a candidate content item for presentation to the viewing user at the placement position.