Instead of

DriverManager.registerDriver(new oracle.jdbc.driver.OracleDriver());

use

Class.forName("oracle.jdbc.driver.OracleDriver");

as this loads the class  explicitly - see

java.sun.com/.../driverma
nager.doc.html

This will also let you know if the driver  can't be found.

Also check that the path to the jar containing the Driver is in the
Java runtime classpath, something like

java -cp .;%ORACLE_HOME%/ora81/jdbc/lib/classes111.jar MyClass

(replace %ORACLE_HOME% with the oracle  folder)