This payment method needs slight patching to work with OneStepCheckout .
The patch is available as diff from https://gist.github.com/speedupmate/7bcdbb8c6af85a54700f and it relates to updating/adding following files
app/design/frontend/base/default/layout/ops.xml (backup before you replace or change) js/netresearch/ops/osc_payment.js (needs to be added) js/netresearch/ops/payment.js (backup before you replace or change)
If you add this patch then you need to observe that after adding the oscpayment.js is actually loaded before payment.js in your browser as oscpayment.js needs to be loaded before to be able to wrap the methods defined in payment.js before so additional functionality can be injected. If inclusion order is wrong you end up in race condition and the patch is not applied. If you can't get this working in right order then you can always rename js files with prefix numbers and then those are included in right order ex: 01oscpayment.js , 02_payment.js
Second issue can affect the Ideal payment method where with Internet explorer order placement redirects to empty cart instead of payment gateway. This can be solved by editing app/code/local/Idev/OneStepCheckout/Block/Checkout.php and replacing lines 936 , 937 (in _saveOrder() method) from
$response = Mage::app()->getResponse(); return $response->setRedirect($redirect);
header('Location: '.$redirect); exit();
Please contact support if you have any other issues with this payment method or patching is too complicated process for you and we can help you out.