Download the jar and place it in your project src/lib. Now you can use the maven installer plugin.
<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-install-plugin</artifactId>
    <version>2.3.1</version>
    <executions>
        <execution>
            <id>install-oracle-jdbc</id>
            <goals>
                <goal>install-file</goal>
            </goals>
            <phase>clean</phase>
            <configuration>
                <groupId>com.oracle</groupId>
                <artifactId>ojdbc6</artifactId>
                <version>11.2.0</version>
                <packaging>jar</packaging>
                <generatePom>true</generatePom>
                <createChecksum>true</createChecksum>
                <file>${project.basedir}/src/lib/ojdbc6.jar</file>
            </configuration>
        </execution>
    </executions>
</plugin>
Now you only have to execute mvn clean once and the oracle lib is installed in your local maven repository.