If there are some issues with how credit card expiry date fields are displayed, then it is most likely an issue caused by your custom theme's CSS. You can solve it by adding the following code to the onestepcheckout.css file:

.v-fix {
width: 49% !important;
}

The onestepcheckout.css file is located in '''skin/frontend/base/default/onestepcheckout/onestepcheckout.css'''. You should copy this file to your theme's local skin directory to avoid having it overwritten when upgrading OneStepCheckout.