Magda is one of my favorites, given it helps you turn the Treasure into artifacts or Dragons two things an Osgir deck is likely to care about a lot
Methods of altering spell costs are deeply ingrained in the game, with examples ranging from Force of Will to mechanics like Affinity, Improvise, and Delve
Honorable Mention Verdeloth the Ancient While the Treefolk creature type originated all the way back in Alpha , this kindred collection didnt receive its first lord (aka creature that boosts another creature type in some meaningful way) until Invasion released in 2000 with this legendary 4/7 Treefolk
Otherwise, it seems harmless
Shimmer Dragon and Workshop Elders As you can see, a common theme among these decks is the lack of finishers
Even something as simple as Syphon Soul would let a 6-mana spell cascade in a 4-player game