well based on my own knowledge of this sect....
Int increases the dmg.
Obs increases the chance of crit of your spells.
Now the ration u should go is maybe... 3 Int : 1 Obs or 4 Int : 1 Obs..... Int would give you more constant dmg while Obs gives crit which gives you dmg bonus(based on your base dmg).
Having too high an Ob would also be useless as the modification on the based dmg would be very small when u're low lvl...