This is what I found in oracle meta link
Connections from Oracle 7 - client and server (database links) - to and from Oracle 9 are not supported. There is no workaround.
Since last patchset of 9iR2 you receive an error in trying to connect to Oracle 7 using Oracle 9 client.
Due to an iTAR at Oracle Metalink Account: Oracle says that Oracle 7 is desupported and suggests all customers to upgrade at last to Oracle 8 (8.1.7).
Question begs to be asked why 8i client against 9 database?
Unless application dictates/requires, would you not go to latest client?
The reason 8i cllient to 9i database is we have both Oracle 9i/8i servers. We just created a couple of Oracle 9 servers for migrating Oracle 8 apps. We have 100's of workstations that have the 8i cleint that would have to be updated. We would have ODBC connections breaking, the ones connected through the Oracle 8 dll. We will update to the Oracle 9i client, we just are not ready. Users would have to switch their oracle home back and forth.