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;
}