I've been keeping track of the major oAuth issues that developers report to me. I've also hit these a few times myself.
1. The date/time on your machine must be accurate. Make sure you’ve performed a recent sync with a known time source. 2. If you’re trying to call a Twitter API method that will update data, the oAuth setup must be Read/Write on Twitter.com/oAuth. 3. Recheck all of your querystring variables for proper encoding. The method that you use for encoding querystring variables may not be oAuth compliant.