Magento already comes with great support for Google Analytics, and it will let you track all purchases and allow you to use the "Ecommerce" section of Google Analytics which is extremely useful for viewing your statistics and also taking sales and revenue into account. When using OneStepCheckout, you don't have to do anything at all to keep the support for Google Analytics' Ecommerce section.
One Step Checkout 4.5.7 has support for Google Analytics and Universal analytics built in. You can enable sending events in System>Configuration>Sales>OneStepCheckout>
OneStepCheckout will push a event on each dom field change to analytics. Building funnels/reports on top of events is business specific task so we have left this to clients .
You can see events registered by OneStepCheckout Advanced tracking feature from google analytics Behavior > Behavior Flow > Events section.