When I stay on onepage checkout page and product is out of stock and when I click on next step it is redirecting cart page due to produc is out of stock So I want to stop to redirect cart page.
Not sure to understand what you need. If you need to let him proceed to checkout anyway you can enable
System > Configuration > Inventory > Product Stock Options > Backorders.
Allow Qty Below 0 and Notify Customer
If it is not what you are lookig for please add further explaination to your question.