If I'm oc-ing, how obvious does it have to be? Can it be behind my back(almost)? Is it concealed if my arm is in front of it? If I'm open-carrying and I get in a car, is it still open-carry? Where can I find these laws?
You'll get more (and better) answers by posting in the Idaho section. Your answers will also be tailored to Idaho law.
Generally speaking, where OC is lawful, and concealing would be unlawful or require a license, your arm and your holster would not be considered to be concealing your firearm. This is not true everywhere. IIRC, Mississippi considers holstering as concealing. (Silly, huh?)
There is limited Idaho case law that goes into it, but it's all over the place. Unfortunately, most of the case law comes from defendants that were trying to suppress illegal drugs found during a "search incident to arrest," which was based on the officer "noticing" a "concealed weapon" in the car. I know...it doesn't make sense...but that's life in states without constitutional carry.
I can't recall the case offhand, and can find and give the citation if needed, but in one instance, the Idaho appellate court found that a firearm not visible from one vantage point (outside the driver's side window) but visible from the passenger side, was "concealed" because it was not readily visible. (rolleyes).
That's why it's best to keep your sidearm, fully loaded, sitting on the passenger seat, console, back seat, dashboard...wherever, so long as it is easily visible from just about any angle.
OC on the person in a holster is fine as long as the carrier is not obviously attempting to obscure part or all of it, such as with an untucked shirt or coat. The natural hang or swing of the arm is not an attempt to conceal, so don't worry about that.