At a minimum, make sure OSSS-UCX, and a suitable launcher, see Building, are on your PATH in that priority:
$ export PATH=/path/to/install/openmpi/bin:$PATH
$ export PATH=/path/to/install/osss-ucx/bin:$PATH
and test:
$ which mpiexec
/path/to/install/openmpi/bin/mpiexec
$ which oshcc
/path/to/install/osss-ucx/bin/oshcc
You can set up a modulefile (http://modules.sourceforge.net/) to simplify this process and set up other SHMEM environment variables that might be of interest, e.g. something like:
#%Module1.0######
set openmpi_root /path/to/install/openmpi
prepend-path PATH $openmpi_root/bin
prepend-path MANPATH $openmpi_root/share/man
prepend-path PKG_CONFIG_PATH $openmpi_root/lib/pkgconfig
set shmem_root /path/to/install/osss-ucx
prepend-path PATH $shmem_root/bin
prepend-path MANPATH $shmem_root/share/man
prepend-path PKG_CONFIG_PATH $shmem_root/lib/pkgconfig
setenv SHMEM_INFO yes
The details of actually running an OpenSHMEM program with OSSS-UCX will vary greatly depending on your environment, be it LSF, SLURM, Torque, standalone...