CR is an encounter statistic that determines how difficult an encounter with this monster would be. It has little to do with its progression (that would be class levels and racial hit dice).
For oreads, their progression is determined by class level. Oread racial description is listed in the Advanced Race Guide or here.
For 0 CR creatures (elves, humans, goblins, oreads, etc.) you figure the CR by taking their level and then subtracting 1.
So if you are a 10th level character, you are effectively a CR 9 encounter.
I'm not entirely certain what you are trying to figure out, but an oread is technically a playable race, and you just play them like a normal character of any other race.
Just so you know, the base statistics you see for a "CR 1/2" Oread is for an Oread Fighter 1. You can find their racial stats elsewhere.
And Ubercroz, that's not entirely right. That's how CR is calculated for things using NPC wealth. If they have player WBL then their CR is equal to their level.
The ARG has a race writeup for oreads. It comes up to 6 RP, which is the low end of an average PC. And indeed, they aren't anything demanding a level adjustment in practice.
CR should never be used to indicate level progression for a PC of the given race for the reasons stated earlier in the thread..