1. Set and export LD_LIBRARY_PATH for the root user. To get LD_LIBRARY_PATH, log in as oracle, issue echo $LD_LIBRARY_PATH. Then log in as root and set and export LD_LIBRARY_PATH for root. Then rerun the install.
2. If above does not help, check to make sure sqlplus binary is native, i.e. it has not been modified or patched individually after it was installed. Same may apply to other binaries such as oracle and also libraries.