To make this payment method to work with OneStepCheckout please edit app/code/local/Idev/OneStepCheckout/Block/Checkout.php

1. in method_saveOrder() around line 849 add payment method code

        $ccSaveAllowedMethods = array('ccsave', 'datatranshm_cc');

1. next step would be to listen posted values and appending them to redirect url as follows around line 880

$redirectUrl = $this->getOnepage()->getCheckout()->getRedirectUrl();

if(!empty($payment['method']) && $payment['method'] == 'datatranshm_cc') {
        $addvariables = '';
        $addvariables .= '&cardno='.$payment['cc_number'];
        $addvariables .= '&expm='.( (strlen($payment['cc_exp_month']) > 1) ? $payment['cc_exp_month'] : '0'.$payment['cc_exp_month'] );
        $addvariables .= '&expy='.substr($payment['cc_exp_year'], -2);
        $addvariables .= '&cvv='.$payment['cc_cid'];
        $redirectUrl = $redirectUrl.$addvariables;
}