So i bought the Nissa vs Ob Nixilis duel decks and I was wondering about this interaction before I decided to build a commander deck from the two... {Ob Nixilis Reignited}'s +1 states I draw a card and lose one life. If I comboed that with the enchantment {Abundance} would I still lose the life despite {Abundance} allowing me to replace the draw?
Yes. You replaced the draw, but the life loss is part of Obs ability resolving.
It would be different if he read something like
"Draw up to One card, lose one life for each card drawn this way".