if i have a creature enchanted with Curious Obsession and my opponent controls a Narset, parter of veils, if my creature does combat damage to said opponent, does curious obsession trigger goes to the stack or narset purely negates it, cause i ve already draw my card for this turn?
Yes, the trigger goes on the stack. The game doesn't look into the future and it doesn't assume that Narset will still be there when the trigger resolves. For the game the Now matters. So until the trigger resolves and the player has to decide to draw or not, Narset's presence is irrelevant. But since Narset is present at that time, the choice to draw cannot be made if a card was drawn already that turn by that player. If Narset is gone at that time, her ability doesn't matter anymore, and the player can draw a card.
if my creature does combat damage to said opponent, does curious obsession trigger goes to the stack or narset purely negates it, cause i ve already draw my card for this turn?
Obsession triggers, and resolves, but you still won't draw a card, because Narset forbids it.
As it resolves, Obsession offers you a choice: to draw, or not to draw.
Since Narset forbids it, it would be illegal for you to choose the 'to draw' option; you must choose 'not to draw'.
608.2d If an effect of a spell or ability offers any choices other than choices already made as part of casting the spell, (...). The player can’t choose an option that’s illegal or impossible (...)
Even if you were not offered the choice, such as with Betrayal, for example, which orders you to draw a card (it doesn't say 'you may'), you still couldn't draw, because:
101.2. When a rule or effect allows or directs something to happen, and another effect states that it can’t happen, the “can’t” effect takes precedence.
On the other hand, under the specific scenario where Narset would die at the same time, due to combat damage, for example, then you will draw.
(this appears to be unwritten assumption of previous posters)
Former Rules Advisor
"Everything's better with pirates." - Lodge
(The Gamers: Dorkness Rising)
"Any sufficiently analyzed magic is indistinguishable from science."
(Girl Genius - Fairy Tale Theater Break - Cinderella, end of volume 8)
As it resolves, Obsession offers you a choice: to draw, or not to draw.
Since Narset forbids it, it would be illegal for you to choose the 'to draw' option; you must choose 'not to draw'.
Even if you were not offered the choice, such as with Betrayal, for example, which orders you to draw a card (it doesn't say 'you may'), you still couldn't draw, because:
On the other hand, under the specific scenario where Narset would die at the same time, due to combat damage, for example, then you will draw.
(this appears to be unwritten assumption of previous posters)
RULES OF MAGIC :
http://magic.wizards.com/en/game-info/gameplay/rules-and-formats/rules