The dominant mode of millennial scale glacial climate variability is governed by Dansgaard-Oeschger (D-O) oscillations, which occurred quite regularly during Marine Isotopic Stage 3 (MIS3). I will present some concepts from dynamical systems theory to illustrate how different timescales in the system produce oscillations of relaxation form and further connect this simple system with the more complex climate system. The fast and slow timescale processes involve a number of climate system components. The fast timescale is mostly determined by processes that involve thermohaline instability in the North Atlantic during stadial to interstadial transitions. The slow timescale is determined by processes that involve interactions between sub-polar gyre convection and Arctic sea ice export along the Denmark Strait (interstadial) as well as gradual changes in the salinity of the sub-tropical gyre (stadial). The modelled interactions between the various components of the Earth system (ocean, atmosphere, land and sea ice system) would therefore characterize the D-O oscillation as a coupled hydrological-thermohaline oscillator.