Depending on the version of OpenMPI you have, either ompi_info --param all all (older versions) or ompi_info --all (newer versions) dumps the full list of MCA parameters available.
The default values and their source are shown in the list and most of the parameters are also documented. Some MCA parameters only become available if certain other parameters are set.
For example, the parameters that control the selection of algorithms for the collective communication operations in the tuned module only become available if one set coll_tuned_use_dynamic_rules to true. To have ompi_info list those too, --mca coll_tuned_use_dynamic_rules true has to be passed to it.
To have all MCA variables dumped the moment MPI_Init() is called, set mpi_show_mca_params to all. The value of each MCA parameter and where that value comes from are then dumped to the standard error stream.