Obsidian Oozes are downright mean. Oozes only get +1 CR per +4 HD and fractions round down, but a size increase would be +1 CR. I'd stat them out at 18 HD Huge (CR 11), 16 HD Large (CR 10), and as-printed (CR 9), and use three to five per encounter for seven PCs but never more than one huge ooze per group. I think the 'Skewer' special attack on its stat block should actually be changed to 'Sharp Spines' based on its ability descriptions. These things will completely destroy anything that's not outright immune to fire.