Well first lets look and the standard checkout procedure in oscommerce 2.2 Millstone:

Step 1. Login or create account choice page
Step 2. Create Account Page
Step 3. Account created confirmation page
Step 4. Shipping choice page
Step 5. Payment choice page
Step 6. Order overview and confirmation order page
Step 7. Thanks for order page.

To shorten this 7 steps procedure there are a few paths/modification you can make.

1. Combined Login and Checkout Page

Combined login and create account page reducing 2 steps.
Simple mod which gives you a login box on top of your create account page. Pre-modded files and screenshot included in the package.

2. Purchase without an account (PWA)

Allow your customers to choose the three following options at checkout time:
1)Proceed Directly to Checkout (no account creation)
2)Create a New Account
3)Login to Existing Account

3. Fast and easy checkout (FEC)

Fast and easy checkout combines the checkout shipping and payment pages in one
combines login and create account to one page and adds checkout without account capability to it.

4. Alternative Purchase System v2.0

Alternative Checkout System that able to change regular checkout process to something more easy and more short with this System your buyers will able to do purchase by couple clicks.

For test go:http://www.procreator.info and then Test Shop

5. More Logical Checkout v1.0

This mod streamline the checkout process with an aim to make it more logical it also removes 2 steps from the checkout procedure by skipping the login/create account page and going straight to create account info page and by removing the create account success page.

Hope you find what your looking for.

Cheers