ORA-01034: ORACLE not available; ORA-27101: shared memory realm does not exist

If oracle database listener and database was started successfully and you're getting following error:
ORA-01034: ORACLE not available
ORA-27101: shared memory realm does not exist



either by connecting through sqlplus or SQL Developer, check if ORACLE_HOME, ORACLE_BASE and ORACLE_SID variables are correctly set.
Also you might want to check if ORACLE_HOME and ORACLE_BASE are not pointing to a symlink (or if there's a symlink in path), as oracle doesn't know/support symlinks.