Links to past and current Oracle documentation:

Oracle 8.0.6
http://www.oracle.com/technetwork/documentation/oracle8-090636.html

Oracle 8.1.7
http://www.oracle.com/pls/tahiti/homepage

Oracle 9.0.1
http://www.oracle.com/pls/db901/homepage

Oracle 9.2.0
http://www.oracle.com/pls/db92/homepage

Oracle Database 10.1.0
http://docs.oracle.com/cd/B12037_01/index.htm

Oracle Database 10.2.0
http://www.oracle.com/pls/db102/homepage

Oracle Database 11.1.0
http://docs.oracle.com/cd/B28359_01/index.htm

Oracle Database 11.2.0
http://docs.oracle.com/cd/E11882_01/index.htm

Oracle Database 12.1.0
http://docs.oracle.com/database/121/index.htm

_See also: https://blogs.oracle.com/UPGRADE/entry/oracle_database_documentation_from_past_