For OneStepCheckout versions < 4.5.*
To fix the issues with IE10 please replace the skin/frontend/base/default/onestepcheckout/js/window.js file with following file https://gist.github.com/speedupmate/5857348 this will fix the opacity and iframe related issues in IE10.
This fix will be bundled starting from OneStepCheckout ver 4.0.10 , current modal library will be replaced completely in ver 5.0
Additionally to that lot of our client site still use old and outdated Magento versions that bundle old prototype.js library that does not work with IE9 (anything lower than prototype.js 1.7 not supporting IE9) and IE10 and also can cause issues that are hard to spot. So it is always advised to stay on top of upgrade wave with your Magento version and also with all your 3rd party versions.