A clamp for attaching a medical device to an OR table rail, including two closable clamp jaws that close to an extreme position that leaves inadequate room for the rail. A handle actuates the jaws apart, and a spring biases them together through a range of jaw positions. A lock member is biased to swing through a range of lock positions in which it progressively engages and locks the jaw mechanism through its range of jaw positions. A pair of bars carries the medical device on the clamp, and allows it to be adjustably positioned with respect to the rail.