If you’ve just installed Oracle 10g XE and want to use that database with Oracle JDeveloper 10g release 3, you might run into a “t2cGetCharSet” error when defining an OCI8 (thick) connection. Copy “lib/ojdbc14.jar” from the XE jdbc-directory to the jdbc/lib directory of JDeveloper. The jar from XE is newer, and seems to solve the problem. You need to restart JDeveloper for the new driver to be recognized/active.
Update: read all about Oracle and JDBC here.