I play Notion Thief in response to a Sensei's Divining Top activation on my end step. My opponent hasn't drawn any other cards on my turn, this activation would be the first card he draws. Will he be denied the drawn card and I draw a card, or no?
Yes. Notion Thief's effect will apply to a card an opponent would draw in parts of the turn other than his or her draw step, even if it's the first card he or she would draw in a turn.
Sensei's Divining Top
I play Notion Thief in response to a Sensei's Divining Top activation on my end step. My opponent hasn't drawn any other cards on my turn, this activation would be the first card he draws. Will he be denied the drawn card and I draw a card, or no?
EDIT (Jul. 24, 2017): Correctness edit.