The limited cognitive abilities of agents that participate in mechanisms has often limited the application of many theoretical results. Obviously strategyproof (OSP) mechanisms have been introduced to address this issue, since they are so simple that even an agent with limited cognitive abilities can recognize that honest play maximizes her welfare. In this work we provide the first characterization of OSP mechanisms for a setting involving agents that have multidimensional types. Specifically, we characterize OSP mechanisms for diffusion auctions, in which a seller has some items to sell, and buyers privately have both a valuation for these items, and a set of neighbors on an underlying graph. The seller needs not only to incentivize each buyer to reveal her valuation, but also to diffuse information about the auction to her neighbors. Our characterization allows to prove that there are OSP diffusion auctions as powerful as some (but not all) of the best known strategyproof diffusion mechanisms in the literature.