From what I can tell, having different MPI package versions on Centos may interfere with each other, which is why they made it such that the /usr/lib64/openmpi*/bin isn’t in PATH.
The best way to install OpenMPI on Centos is:
$ sudo yum install openmpi-devel
$ module load mpi
$ mpifort --version
GNU Fortran (GCC) 4.8.5 20150623 (Red Hat 4.8.5-36)
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