Still not satisfied with the -7, but I can't really think of any way to adapt Oberon's abilities in a more color-fitting manner.
Oberon, Forest Guardian Z@&
Creatures you control get Vigilance, and Hexproof until the beginning of your next turn, You gain 2 life.
'You get an emblem with "Creatures your
'opponents control have their toughne: reduced to half, rounded down".