Please see our blog post on this subject and see also full information below. Blog post is available here: https://blog.onestepcheckout.com/2018/03/do-amazon-pay-and-paypal-express-work-with-onestepcheckout/

One Step Checkout works together with Amazon Pay and Paypal Express, however they cannot be configured as a payment method inside One Step Checkout. Amazon Pay and Paypal Express are checkout modules that work in parallel with One Step Checkout and we recommend adding "Amazon pay" or "Pay with Paypal" button next to all "Go to checkout" buttons in cart, popup mini cart, etc so that this will be the point where the customer makes the choice to check out with one or another.

Both Amazon Payment and Paypal Express Checkout are equal checkout extensions to OneStepCheckout, they bypass One Step Checkout and order placement will be finished in Paypal Express review page or Amazon Payments extension . We recommend having Amazon and Paypal Express buttons on cart page and not on One Step Checkout page. Those are not sole payment methods and to test those you should sign up with their developer or testing programs.