Once the customer adds the product to the cart, admin can enable the auto-redirection from the back-end in Magento 2 One Step Checkout Extension's configuration.

Here are the steps to enable the auto-redirection from back-end when customer adds the product to the cart.

1. Login to Magento Admin.
2. Navigate to Stores >> Configuration >> One Step Checkout >> General Configuration.
   
   
3. Select Yes in the option Redirect to checkout page on Add to cart of product.