Honestly I think Oghren has the advantage, while he may not be able to get at Sten's head and chest easily he can go for something that could prove to be Sten's weakness in a fight (assuming Sten is using a two handed weapon.) his legs.
To clarify when using a two handed weapon you're using more strength and therefore using a weapon like that with greater force, if you're aren't steady on your feet or haven't got your legs positioned correctly you'll be off balance with a heavy weapon and an experienced fighter could exploit that.
Now let's say for example Oghren manages to wound or atleast disable Sten's legs, Sten would be unable to effectively use his weapon against Oghren and more than that Sten is going to have to adjust his fighting style to compensate for the large size difference, Oghren on the other hand is used to fighting Darkspawn and would know how to take down larger opponents without getting squashed.
so personally I think the Drunken Beserker would win this one, but in all honesty I have had the same thing happen to me that mentioned, Oghren does tend to last longer in my parties than Sten does.