All versions of Oracle. Also add a version-specific tag like oracle-11g-r2 if that context is important in the question.
Oracle is a relational DBMS (Database Management System) created by Oracle Corporation. It supports a large number of languages and application development frameworks. Its primary languages are SQL, PL/SQL and Java. When asking a question with the oracle tag, please be sure to include the full version number, (i.e., 10.2.0.5, 22.214.171.124), in addition the relevant version-related tags described below.
Oracle makes plethora of documentation available at http://docs.oracle.com.
Questions about SQL should be tagged sql. If the question applies to an Oracle database it should probably also be tagged oracle.
Questions about PL/SQL should be tagged plsql as well as oracle.
Questions specific to a particular database version may be tagged with a version tag such as oracle11g-r2, oracle-11g, oracle-10g or oracle-9i, but they should also be tagged oracle.
Questions applicable to 126.96.36.199 through 188.8.131.52 should be tagged both oracle-11g and oracle.
Questions applicable to 184.108.40.206 through 220.127.116.11 should be tagged both oracle-11g-r2 and oracle.
Questions applicable to other versions should be tagged as oracle-10g, oracle-9i or oracle-8i as well as oracle.