Yes, Ohm's law is an approximation that is reasonably accurate for many materials (such as the wire in wire-wound resistors), but not a fundamental law of electrodynamics. For two-wire components listed as "resistors" in catalogs, the deviation from the proportional relationship between V and I is called the "voltage co-efficient of resistance", which is rarely specified on data sheets. I have found substantial deviations in high-resistance, short-length components such as 0805-size 50 megohm parts at 10 V. The high-resistivity materials needed to obtain that resistance, with the high voltage gradient (V/mm) along the path suffer from the non-linear behavior of the actual material.
In physics, in the linear limit of small voltage gradients, the law is stated as
J = (sigma) E
where the vector J is the current density (A/m2), the vector E is the electric field or voltage gradient (V/m), and (sigma) is the conductivity tensor which may be interesting for an anisotropic medium (such as crystalline graphite).