In this paper we present OPSS, an Overlay Peer-to-peer Streaming Simulator designed to simulate large scale (i.e. in the order of 100K nodes) peer-to-peer streaming systems. OPSS is able to simulate a fair (i.e. "TCP-like") sharing of the uplink and downlink bandwidth among different connections, and it guarantees extensibility by allowing the implementation of different peer-to-peer streaming algorithms as separate modules. Source code of OPSS is available under the GPL license. © 2018 International Conference on Learning Representations, ICLR. All rights reserved.