You can accept payments on your OSCommerce enabled website using Paypal Express. The below tutorial explains the detailed workabout to implement this.
Step 1: Make sure you have your OSCommerce Website up. Login to the admin section. The URL usually is yoursite.com/admin/login.php ( Unless you have changed it)
Step 2: On the left hand side of the dashboard, lookout for Modules-> Payment.
Lookout for Paypal Express Checkout. In case this is not available. Check the Install Module box at the right hand top. Install Paypal Express Module from there. Once installed, click to edit it.
Step 3: Enable the module by selecting, ‘true’.
Fill in your paypal email Id in the Seller Account.
Step 4: Now for Live purpose, you have to select the Live Server and enter the API details. However, if you are currently testing the site with dummy monetary transactions, select the test server and enter the Test Server API Details.
The test account APIs can be created at the sandbox site. Visit http://developer.paypal.com/
Once the API Username, API Password and API Signature are entered, you are live to go. Make sure, these API details are kept confidential.
Best luck with your code play !