Action space synthesizes the orbital information of stars and is well-suited to analyse the rich amount of kinematic disk substructure in the Gaia DR2 radial velocity sample (RVS). In this work, we revisit one of the strongest perturbers in the Milky Way (MW) disk: the m=2 bar. We investigate how its resonances affect the actions of individual test particle stars, i.e., (JR,Lz,Jz) estimated in an axisymmetric MW potential. We confirm that the stars' behaviour is well approximated by scattering and oscillation along a slope Delta JR / Delta Lz = l/m centered on the l:m resonance lines. The Outer Lindblad Resonance (OLR, l=+1,m=2) creates signatures in the stellar action space that can be used to identify the Galactic bar's OLR in the Gaia DR2 RVS data: (a) The JR dependence of the oscillation causes an overdensity ridge (underdensity region) at Lz larger (smaller) than the resonance line in the (Lz,JR) plane. (b) For the first time, we demonstrate that the OLR is expected to cause a gradient in average Jz with Lz across the resonance. (c) We show that the change of predominantly outward to inward motions at the OLR occurs along the resonance line in action space. The latter signature allows us to identify three candidates for the bar's OLR - and therefore its pattern speed Omega_bar - in the Gaia data within 3 kpc from the Sun: 1.85 Omega0, 1.2 Omega0, and 1.6 Omega0 (with ~0.1 Omega0 uncertainty). This demonstrates that (i) the local Gaia action data is consistent with both the short-fast and long-slow bar models in the literature, and that (ii) axisymmetrically estimated actions are a powerful diagnostic even in non-axisymmetric systems.