Game Guide - Activating the DLC Dragon Age: Origins - RtO Guide

After you've installed this DLC you can load a saved game and you'll be informed about the possibility of starting a new quest. Important! You won't be allowed to move on with this mission until you've left [Lothering] and therefore completed Lothering and the imperial highway main quest! You'll also have to start it BEFORE travelling to [Redcliffe] to take part in The final battle mission.

An important thing about Return to Ostagar is that the difficulty level of this DLC will change depending on your current progress. I would recommend starting this quest after you've successfully returned from [The Circle tower], because you'll want to have Wynne (as a healer) in your team. Don't forget to free some space in your inventory, because you'll have a chance to collect a lot of useful things in the near future, including King Cailan's full armor set.