I believe that on Centos the correct way to access MPI is to use the "environment modules" system to load the openmpi module. When you installed OpenMPI through the "yum" interface it should have installed the "environment modules" system *and* a module file for mpi.
Type "module avail" and it should list off all the optional modules installed. You should see something like "mpi/openmpi" (modulo suffixes). If so, you would then type "module load mpi/openmpi" (or whatever you found in "module avail").
The environment modules system is set up to allow you to swap optional packages in and out of your path with less effort than it would normally take.
I am not *certain* that Centos does it that way, but it is definitely how it works on RHEL, which is basically the commercial analog of Centos.
Certainly, specifying the absolute path will get you at least part of the way, and the way OpenMPI works it should make sure the resulting binaries find everything they need. But in general, if an environment module exists for a package it is best to use it because they usually do more than just add to your PATH (they sometimes add to other environment variables that are critical to the package's operation).