Behaviour of a capacitor+resistor (frequency domain)
why a capacitor behaves as an open circuit when \$\omega&lt;\omega_p\$
  and as a short circuit when \$\omega&gt;\omega_p\$ (where
  \$\omega_p=1/RC\$ is a pole)?


It doesn't.

Q=CV and differentiating to obtain current we get I = C dv/dt. Basically the current that flows is proportional to the rate of change of voltage. It's as simple as that.