Need help with access of DB problem

Not sure if we have an Oracle problem or an O/S problem but would appreciate any help that can be offered.

We have installed Oracle 9.2.0.6 on a Red Hat Linux 3.2 32-bit O/S.
We can only access DB with SQL*plus that is started by Oracle process owner.
Once Oracle is started, the user that started the database can access the database using SQL*plus.
Any other user with the same privileges cannot get in.
We get:
ERROR:
ORA-01034: ORACLE not available
ORA-27123: unable to attach to shared memory segment
Linux Error:  13:  Permission denied

We have beat ourselves up for two days and are no further along.
All help is much appreciated.