Hey guys quick question. If I have Omniscience in play and I play Tooth and Nail and I want to get the extra ability of Tooth and Nail do I cast it for 0 mana and pay the Entwine cost of 2 mana or do I have to pay its CMC plus the Entwine cost? Thank you.
Entwine expresses an additional cost (C.R. 702.41a). And effects that let you cast a spell "without paying its mana cost", as in Omniscience, don't affect additional costs such as entwine (C.R. 117.9, 117.9d). Thus, in general, if you cast Tooth and Nail from your hand while controlling Omniscience—
and choose one of that spell's modes (not: abilities), you pay nothing (C.R. 608.2f).
and choose both of that spell's modes, you pay only 2, which is Tooth and Nail's entwine cost (C.R. 608.2f, 702.41a).
EDIT (Jun. 11): Add rule citation.