Return of Reckoning

So is Oppressor really worth the SC grind?
or should i try to complete Dominator? (currently 3/6) and then focus on Weapon? (offensive BG)

Well, all set's can be worth grinding out. If it's the offensive ones (Like you stated) then yea oppressor is one of them, but it's not a must. you do fine still with a standard Dom/Bl/merc combo or even full set of one if you would like. Take a look at sentinel aswell, it to is a nice offensive one.