Yes you right and that was my problem, after a long research I have found the solution

I was closing the underlying connection and never the native connection which was always active, meaning that the pool was exhausted very quickly

to help people with the same problem

add this method to retrieve the underlying OracleConnection :

    import java.lang.reflect.Method;

    public static OracleConnection getOracleConnection(Connection conFromPool) throws SQLException {

        try {

            Class[] parms = null;

            Method method = (conFromPool.getClass()).getMethod("getUnderlyingConnection", parms);

            return (OracleConnection) method.invoke(conFromPool, parms);

        } catch (InvocationTargetException ite) {

            throw new SQLException(ite.getMessage());

        } catch (Exception e) {

            throw new SQLException(e.getMessage());

        }

    }

in the services that needs to use Oracle Data Objects :

        Connection conn = null;

        OracleConnection oraConn=null;

        OracleResultSet rset = null;

        OracleCallableStatement stmt = null;

        conn = this.getConnection(ctx);

        oraConn = getOracleConnection(conn);

// do your work using oraConn

// close the native connection

    conn.close();

     conn=null;