The Wizard of Oz event - the only Opera we ever see in Karazhan with any regularity. Below I present my guide to it, the only reason I'm writing this is because I got my hands on the perfect illustrations for each boss. You'll see what I mean.
The Oz event starts with you facing off against 4 bosses; Strawman, Dorothee, Roar and Tinhead. These bosses activate in the order: Dorothee, Roar, Strawman, Tinhead, and Dorothee will summon her dog Tito at some point. Basically this fight is about following the kill order and certain classes get to do some specific things to help out. Let's look at each boss.
Dorothee has no aggro table, so she attacks whoever she fancies with Icebolts. After a while she will summon her dog Tito. Basically we nuke Dorothee as fast as possible and don't tank her; when Tito spawns, a hunter will put their pet onto him to tank him - Tito is the next to die. Having a somewhat tough pet helps her, as does keeping Mend Pet up and using Intimidation to stun, if you have it.
Roar is a cowardly lion who hits quite hard on cloth. Because he is cowardly, he can be feared easily, so we generally will get a tank to hit him a couple of times to build some threat, then get a warlock to chain fear Roar around the place. Roar spends most of the fight being feared and is killed after Strawman. A hunter can also use Scare Beast in case the Warlock's fear is resisted and Roar is massacring the clothies.
Strawman, being made of straw, is very susceptible to fire damage. We usually assign a tank to Strawman who will drag him somewhere and keep him busy. However, when we have been short a warlock, we have put our 2nd tank onto Roar, so a mage has had the awesome task of keeping Strawman under control. Because any fire damage has a good chance to disorient Strawman for 6 seconds, a mage can keep him locked down permanently with Fire Blast and Fireball. Usually Strawman is the third to die, right after Dorothee and Tito.
Tinhead hits very hard and so is tanked by the main tank. After a while he rusts and so the tank can actually kite him around the back of the stage, resulting in the tank not taking too much damage. We usually therefore put the Bear onto tanking Tinhead and Tinhead is the last to die. I say last but actually what I mean is....
Once all the travelling companions are dead (killed by some evil loot-seeking raiders), The Crone appears after a short break, short enough for us to start drinking to restore mana. This is a tank-and-spank fight, apart from the large cyclone which whirls around the stage. The tank and the raid need to move around to keep themselves out of the cyclone - anyone in the cyclone is whipped into the air and takes fall damage. The Crone has also learnt some Shaman tricks - she periodically does a chain lightning attack which can be mitigated by a Nature Resistance Totem or a hunter Aspect of the Wild.
Having not actually healed this I can't give any massively useful tips. Dorothee will damage anyone so I would expect to be on the look out for random people taking damage. Roar is nasty if he gets lose into the clothies. Tinhead hits hard initially but after a while his tank should be taking little damage. Strawman, if tanked by a mage, should be locked down but the mage might appreciate a watchful eye just in case he gets to them. The pet tanking Tito is most likely in need of a heal; Mend Pet just isn't enough to keep a normal DPS pet (such as a cat) tanking Tito with its face, however if this isn't possible a pet will keep Tito busy for long enough that the raid should be able to kill Dorothee and then focus fire on Tito. The hunter won't be happy with you tho :) The Crone does that chain lightning which hits multiple people so more raid damage, although any nature resistance buff seems to mitigate a lot of it. Finally if anyone is silly enough to get caught in the Cyclone they will probably take fall damage, so again more multi-person damage. Really see the Hoof n'Healz guide to Oz for a good guide.