If I normally poison on a 6+, and have some sort of buff that adds +1 to hit, can I poison on an unmodified roll of 5? (Modified roll of 6)
I would say no. On page 73 of the BRB it states that: "A model with the Poisoned Attacks special rule wounds his target automatically if his natural dice roll to hit is a 6". The reference to a natural roll of 6 indicates that the dice must physically show 6 in order for poison to take effect.
but are there not ways you can improve your chance to hit with poison attacks from a 6+ to a 5+ somehow? or is that only restricted to OnG?
Blades of Putrefaction from the Lore of Nurgle can also improve poison so that it takes affect on a 5+.
Nope, I do not believe that there is a way... at least under 8th edition rules, you can do it in 9th Age.
For what's it's worth. You can never get poison on a "7" to hit, so lowering a "7" to a natural "6" means more poison. That's actually useful for blowpipe Skinks. Since moving, multiple shots, long range is 7 to hit (that's why I prefer javelin Skinks which are usually 5+ to hit.
Tehenhauin has poisoned attacks (and is immune to poison himself) but he does not poison on a 5+. As far as I can tell, Tiktaq'to doesn't have poisoned attacks at all.
I must be getting my editions confused... Edit: doh of course it was ... Oxyotl Master Predator - If Oxyotl remained stationary in his preceding Movement phase, shots made from the Golden Blowpipe of P'Toohee in the Shooting phase wound on any successful To Hit roll of a 5+. *(poison shots) I knew somebody had a 5+ poison...
That makes sense. I completely forgot about that guy. He is so overpriced I've never even considered to put him into one of my lists.