fuser not in the PATH when apply OPatch

I was trying to apply Oct CPU 2006, loaded OPatch utility to ORACLE_HOME and unzipped it. And set evn for ORACLE_HOME and PATH. When apply it, I got the following message.

Problems when checking for files that are active.
There were problems when checking for active processes on critical files.
The patch tool runs the command 'fuser' to check that critical files are not in use. Please check 'fuser' is available in your PATH environmental variable.
ERROR: OPatch failed during pre-reqs check.

what platform?

HP-UX11i PA-RISC

On HPUX boxes the fuser binary is not world-executable. As the root user copy /usr/sbin/fuser to $ORACLE_HOME/bin and chown oracle:oinstall $ORACLE_HOME/bin/fuser (assuming Oracle software owner and group are oracle and oinstall respectively)