Ohmic law question?

Ohm's Law is not actually a law, it is a approximation in small regions of temperature. You can imagine that a graph of the resistance against temperature is not exactly a straight line but actually a curve. If you 'zoom in' on a small region of temperature however, the curve is almost a straight line. In everyday life Ohm's law has proven to be very useful, in other words, exceptions (at room temperatures) are rare.