Play half of the octaves only with your thumb, relax your wrist. Complete the octave,the wrist should remain as unchanged as possible. When playing octaves focus on the thumb not the other finger. You shouldn't have to about the other finger, some people lock a shape with their thumb and other finger (4th or 5th usually), they relate the thumb to the other finger and other finger to the thumb with equal importance. We should however give much more bias to the thumb and consciously consider it, the other finger follows it, it is not the other way around or with equality of consideration, the placement of the thumb is the control tower. These subtle things in our hands are near impossible to discuss in words alone, you really need to see the persons hands, physically move their hand and adjust it, demonstrate the subtle difference of right and wrong in their own approach, push towards efficiency through physical training.