1. This law is valid to conductors only at constant temprature. The resistance of a conductor increases with the increase in its temperature. Hence, the V-I graph for a conductor would not be a straight line.
2. Insulators do not conduct electricity at all, so the Ohm's law is not applicable to the insulators in some cases.
3. Ohm's law is also not applicable for the semiconductors, as the material begins to conduct at certain voltage which will give a steep rise to V-I graph and make it non-linear.
4. This law is not applicable to the unilateral networks(has elements such as diode or transistors etc; which do not have same voltage current).