Eldrazi Tron leans on colorless artifact and Eldrazi ramp (Mind Stone, Expedition Map, Chalice of the Void, Karn, the Great Creator), so Obsidian Charmaw comes down cheaply against their many colorless-producing lands and can destroy a key nonbasic Tron piece like Urza's Tower or Eldrazi Temple, disrupting their mana. Vexing Bauble counters spells if no mana was spent to cast them, blunting zero-mana plays while giving you a proactive answer to spend mana efficiently around. Nethergoyf and Ox of Agonas are both weak against their large colorless bodies (Thought-Knot Seer, Devourer of Destiny) and get outclassed on the ground, so they're trimmed for the land-destruction plan.