Seems like you are missing some environment variables.
ORACLE_HOME = {oracle_installation_path}
LD_LIBRARY_PATH = $ORACLE_HOME\lib
PATH=$PATH:$ORACLE_HOME\bin
You should put this into .profile for oracle user, so they get setted on oracle's user login.
For example, if Oracle is installed in /app/oracle/product/10.2.0/db_1
export ORACLE_HOME=/app/oracle/product/10.2.0/db_1
export LD_LIBRARY_PATH=$ORACLE_HOME\lib
export PATH=$PATH:$ORACLE_HOME\bin
The .profile file is located under the user home directory, in this case /home/oracle
If the file doesn't exist, yo can create it
touch .profile
and then edit an paste the values, setting ORACLE_HOME depending on your installation path.
save the file, and give it execution permissions
chmod +x .profile
Then logoff and login again, and you should be able to run it.
Also you can add the environment vars to .bash_profile which is also located on user home directory. If it doesn't exist, create it and do the same as with .profile