From the advisory
"The attack starts with the attacker logging into an account he owns at the (honest) Consumer site. The attacker initiates the OAuth authorization process but rather than follow the redirect from the Consumer to obtain authorization, the attacker instead saves the authorization request URI (which includes the Request Token). Later, the attacker convinces a victim to click on a link consisting of the authorization request URI
to approve access to the victim’s Protected Resources to the (honest) Consumer.
By clicking on the link, the victim continues the request that the attacker initiated, including the Request Token that the (honest) Consumer issued to the attacker. Note that the victim is redirected to the legitimate approval page at the Service Provider and prompted by the Service Provider to approve the (honest) Consumer. It is not possible for the victim to detect that there is an ongoing attack.
After the victim grants approval, the attacker can use the saved Request Token to complete the authorization flow, and access whatever Protected Resources are exposed by the (honest) Consumer site as part of its service."
Advisory Link: https://oauth.net/advisories/2009-1
Additional reading: https://www.hueniverse.com/hueniverse/2009/04/explaining-the-oauth-session-fixation-attack.html