Mine Obsidian by pouring water onto lava
Use a Nightmare Pickaxe or better to mine it
Find lava pools underground or in the Underworld
Bring a bucket or water source to create Obsidian
Craft an Obsidian Skin Potion if you need to stand in lava safely
Open Obsidian Crates for a chance to get Obsidian
Check Shadow Chests and underground loot for Obsidian-related items
Use an Obsidian Skull or Obsidian Rose if you mean Obsidian gear or accessories
Craft Obsidian from lava and water if you need more of it quickly