We consider the problem of designing an efficient oblivious transfer (OT) protocol that is provably secure in a concurrent setting, i.e., where many OT sessions may be running concurrently with their messages interleaved arbitrarily. Known OT protocols use zero-knowledge proofs, and no concurrent zero-knowledge proofs are known that use less than a poly-logarithmic number of rounds (at least without requiring a pre-processing phase, a public random string, an auxiliary string, timing constraints, or pre-distributed public keys). We introduce a model for proving security of concurrent OT protocols, and present a protocol that is proven secure in this model based on the decisional Diffie-Hellman problem. The protocol is efficient, requiring only a slightly non-constant number of rounds. Index Terms: protocols; cryptography; timing; concurrent oblivious transfer; protocol; concurrent setting; zero-knowledge proofs; poly-logarithmic number; public random string; auxiliary string; timing constraints; decisional Diffie-Hellman problem