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Open string theories can be decoupled from closed strings and gravity by scaling to the critical electric field. We propose dual descriptions for the strong coupling limit of these NCOS (Non-Commutative Open String) theories in six or fewer spacetime dimensions. In particular, we conjecture that the 5-dimensional NCOS theory at strong coupling, is a theory of light Open Membranes (OM=), decoupled from gravity, on an M5-brane with a near-critical three-form field strength. The relation of theory to NCOS theories resembles that of M-theory to type-II closed string theories. In two dimensions we conjecture that supersymmetric U (n) gauge theory with a unit of electric flux is dual to the NCOS theory with string coupling 1/n. A construction based on NS5-branes leads to new theories in six dimensions generalising the little string theory. A web of dualities relates all the above theories when they are compactified on tori.