After you install oracle your oraenv script should be located in /usr/local/bin/oraenv. Verify that that is the case and make sure that /usr/local/bin/oraenv is inluded in the path.
Also make sure that oratab file normally located in /etc has entries
for all databases with properly set oracle_home:

type . oraenv and you will be prompt to enter ORACLE_SID if you set
ORAENV_ASK to "YES"