Found this summary on another site, hope it helps
This is a hole in the OCL language. There is no way to bind the output values of operation calls, except for the single return parameter (I’m still upset that UML removed the multiplicity of returns).
However, it doesn’t restrict the formulation of post-conditions. A
postcondition can read the values of INOUT and OUT parameters. In a
precondition, OUT parameters are undefined.