are airmiles earned through one airline in the oneworld alliance valid for any airline within it? ie, if i earn some mileage with AA can i then use it for a BA flight? i'm new to all this... thank you

You can earn miles for the Frequent Flyer program of the oneWORLD carrier you are flying with. In most cases you can also earn points in another Frequent Flyer Program of a different oneWORLD affiliate airline instead.

For example, if you fly on an AA flight you can earn points in AA's AAdvantage program, or, you can earn in BA's Executive club program instead.(some transatlantic flights are excluded).

As far as redeeming goes, and in answer to your question, you can indeed use points earned in one oneWORLD carrier's program to for award travel on another oneWORLD carrier's flights.

The above is a bit simplistic; there are other factors involved such as award availability, earning ratios, etc. You will need to have a close look at the program rules for each individual FF program.

Yes, you can use AA miles for an award flight on BA, except you cannot get an award on BA between the U.S. and the U.K. (you have to pick up BA in Canada, Mexico, or the Carribean). There are two ways to fly other carriers: Partner awards details here and OneWorld awards details here. The basic difference is that Partner awards are designed to get you from here to there and back again, and OneWorld awards are useful if you want to fly around to a lot of different places once you get there.