not able to insert values from a dataset to a database table.

As CHill60 has indicated, you are missing the commit. But you have also not taken advantage of the using statement. In the following, I give the template that I think your method should take.

private void button1_Click ( object    sender,
                             EventArgs e )
    {
    DataSet     data_set = new DataSet ( );
    DataTable   data_table = new DataTable ( );
    string      insert_statement = String.Empty;

    using ( OleDbConnection ole_db_connection =
                                new OleDbConnection ( ) )
        {
        OleDbDataAdapter  ole_db_data_adapter;
        OleDbCommand      ole_db_command ;

        ole_db_connection.ConnectionString =
            "Data Source=XE; " +
            "User ID=ER; " +
            "Password=ER; " +
            "Provider=msdaora";
        ole_db_connection.Open ( );
        ole_db_command = new OleDbCommand ( );
        ole_db_command.Connection = ole_db_connection;
        ole_db_command.CommandText =
            "select * from bill where BILL_NO = '" +
            comboBox1.SelectedValue.ToString ( ) +
            "' ";
        ole_db_command.CommandType = CommandType.Text;
        ole_db_data_adapter = new OleDbDataAdapter (
                                            ole_db_command );
        ole_db_data_adapter.Fill ( data_set );
        }

    data_table = data_set.Tables [ 0 ];

    insert_statement = "INSERT INTO BILL2 ( " +
                            "BILL_NO, " +
                            "CUSTOMER__NAME, " +
                            "ITEM_TAKEN, " +
                            "PRICE, " +
                            "MODE_OF_PAYMENT, " +
                            "TOTAL, " +
                            "DATE_PUR, " +
                            "SALE_FROM ) " +
                       "VALUES ( " +
                            ":BILL_NO, " +
                            ":CUSTOMER__NAME, " +
                            ":ITEM_TAKEN, " +
                            ":PRICE, " +
                            ":MODE_OF_PAYMENT, " +
                            ":TOTAL, " +
                            ":DATE_PUR, " +
                            ":SALE_FROM )";

    using ( OracleConnection oracle_connection =
                             new OracleConnection ( ) )
        {
        OracleDataAdapter oracle_adapter;
        OracleTransaction oracle_transaction;

        oracle_connection.ConnectionString =
            "Data Source=XE; " +
            "Persist Security Info=True; " +
            "User ID=ER; " +
            "Password=ER; " +
            "Unicode=True" );
        oracle_connection.Open ( );
        oracle_transaction =
            oracle_connection.BeginTransaction ( );
        oracle_adapter = new OracleDataAdapter ( );
        oracle_adapter.InsertCommand =
            new OracleCommand ( insert_statement,
                                oracle_connection);

        foreach ( DataRow data_row in data_table.Rows )
            {
            oracle_adapter.InsertCommand.Parameters.Add (
                new OracleParameter (
                    ":BILL_NO",
                    data_row [ "BILL_NO" ] ) );
            // :
            // :
            oracle_adapter.InsertCommand.Transaction =
                oracle_transaction;
            oracle_adapter.InsertCommand.ExecuteNonQuery ( );
            }
        oracle_transaction.Commit ( );
        }

    MessageBox.Show ( "DONE" );
    }
}

Note that the ole_db_connection in the first using need not be closed. And by adding a second using for oracle_connection, there is no need to close it. Just before the end of the oracle_connection using, the oracle_transaction Commit occurs.

Hope that helps.