LM2596 and other DC-DC converters why oscillate if the output capacitor ESR value is too low?
The ESR of the capacitor forms a zero at frequency fz = \$\frac{1}{2\pi\cdot\text{ESR} \cdot C}\$, which tends to reduce the stability excessively if the ESR is too low. The ESR also should not be too high, of course, so usually there is a recommended range. Older parts that were designed before high-capacitance ceramic parts were ubiquitous may omit specifying the lower limit. This is equally true of LDO regulators.  

You can either add some series resistance to the capacitor (maybe a few hundred m\$\Omega\$) to keep it within the recommended range, or use another type of regulator that is designed to be stable with ceramic capacitors. To confirm that you've got it right, you can measure the phase margin, which ideally might be 60 or 70 degrees, for example by injecting a signal and measuring the response.  

If you roll the dice and just throw the cap in there, hoping for the best, you may find that it oscillates at temperature extremes or it will have such a low phase margin that it exhibits excessive overshoot/undershoot.