Everyone makes mistakes. As much as I worked to make AiO the best Oz game out there, some things slipped through. For the most part, it was a lot of extra apostrophes when it came to "its" (Even though there is a nifty song to help you remember). But there was one mistake that could keep people from enjoying the game.
In Chapter 4, the magic rules, most of the chapter is devoted to helping you figure out the Effect Power of a spell, but no specifics on what to do with that number. Well, here it is: The Effect Power of a spell is intended to serve as the penalty on the Brains roll made to cast the spell. So if the Effect Power of a spell is 3, you must succeed at a Brains roll with a -3 penalty to successfully cast it.
I also meant to include an example of spell casting, but I wound up doing some pretty heavy changes to the magic rules late in the process (which is probably why that mistake got through). While adding it back into the book would be pretty heavy work for my layout person (and irritate the people who have already bought it) I can put up an example here.
Let's say that the Wizard of Oz is traveling with a group of friends and the time has come to set up camp. He asks around and gets three handkerchiefs which he will proceed to turn into tents using Transmutation magic. This is Power 3 according to the Transmutation table on page 35 of the book.
Now to figure Duration. Transmutations are usually permanent and have a Duration of 2. But if the caster specifies a way to end the effect, it goes down to 0. Since the Wizard want to make cleanup easy in the morning and is sure his friends want their handkerchiefs back, he designates a magic word that he will say in the morning to end the spell. Duration 0.
The Scope of the spell is 2, as it affects a Group of 3 handkerchiefs.
The Effect Power is currently 5 (3 + 0 + 2), which is pretty high. Which brings us to the last element of the spell: Ritual. This give you a chance to buy down the cost of the spell by making lots of effort in the casting. The Wizard attempts a Poetry Ritual, making a roll against his Presence skill. He has the Poet trait, so he takes no penalty on the attempt. His highest successful result is a 3, providing a Ritual modifier of 3. Poetry rituals are chancy, but this one paid off nicely.
Once all factors are considered, the final Effect Power of the spell is only 2. The Wizard makes a Brains -2 roll (and maybe spends an Oz Point or two) and camp is set up and ready to go.