oo is the distance from the wheel mounting surface to the inside flange (not including the metal bead) of the wheel. Offset is the distance from the wheel mounting surface to the exact middle of the wheel width. Same measurement needs to be made, but after that, you need to do some math to find offset.
Backspacing is a lot easier to measure, a straight edge across the rear flange of the wheel, then a perpendicular measurement to the wheel mounting surface. Offset would need to take the same measurement, then calculate the center point of the wheel width and subtract the wheel flange measurement. For our cars, the backspacing will usually be about half the wheel width, and close to zero offset; so for a 15" X7 " wheel, the backspacing will usually be around 3 1/2", give or take a 1/4" or so, and offset will be around 0 give or take a 1/4" or so.
For most stock rear wheel drive cars, the backspacing will be close to half the width of the wheel (close to zero offset). It can vary some for specific reasons. For front wheel drive cars, the backspacing will be more than half of the width of the wheel, and a positive offset (wheel mounting surface will be closer to the visible or outside wheel flange). Flangeless wheels are the result of very high amounts of positive offset/backspacing. The backspacing/offset seems to vary a lot more for front wheel drive cars.