You'll never oom as a Shadow Priest if you know what you're doing.
People saying not to wand/don't take wand spec started in Cata and have no clue how to level as a Priest.
(EDIT: for clarification, the reason why you want to wand, once you stop casting any spell, a hidden timer starts ticking, which is referred to as Mp5. Once 5 seconds of no casting is up, you start to regenerate your mana. This is why wanding is extremely important, will allow you to survive easily, kill fast especially with the wand spec
)
Generally, at level 10 you'll NEED a wand, this is not a want, it's an absolute necessity.
Lesser Magic Wands do the job until 13 (where greater becomes available with almost double DPS)
Anyways, level 10 to 20 rotation generally goes like this.
(This is what I do, and I never oom).
Mind Blast > Shadow Word: Pain > Smite x1(you can smite an additional time if you feel like a rebel) then wand to death.
At 20 it gets intense. Instead of using smite twice, try and use a Mind Flay. Just to mix things up a bit :>.
Always stack spirit gear, always. (For example, if you see an item with 5 int and 10 spirit. And an item with (5 spirit and 10 int, you'll really want the one with 10 spirit. Reason being is the 0.5% crit from the Int is far worthless compared to the superior bonus your spirit will get with Spirit Tap, trust me. You'll still kill extremely fast and you'll never, ever oom. ) and ALWAYS keep your wand up to date. This is extremely important!
All of that and you should be good! Priests really pick up once you 5/5 Spirit Tap and 5/5 Wand Spec, then continue down the Shadow Tree. ^_^