This paper is motivated by two observations: (1) cellular network operators are actively exploring advertisement delivery as a new means of revenue generation, and (2) cellular basestations perceive intermittent overloads at different times of day. Bringing the two observations together, we design and implement Opal, a novel system for opportunistically aligning advertisement delivery with basestation overload. Such alignment improves the overall perception of network availability for users. To achieve the alignment systematically, Opal builds on an analytical framework for tunable unavailability of network service to users during overload. At the same time, if the network is not overloaded enough during a certain period, Opal schedules enough advertisements to satisfy the advertisement delivery contracts. Opal minimizes the amount of state to be maintained to play advertisements to users uniformly and also maximizes the number of viewers for each advertisement. We implement a prototype of Opal on a Picochip based WiMAX testbed, and demonstrate its efficacy using simulations, analysis and prototype evaluation. © 2011 ACM.