The geomagnetic field is one of the most prominent characteristics of our planet. It protects us from harmful solar and cosmic radiations and provides us with the North and South geomagnetic poles. Most importantly, it opens a window on the deepest region of our planet: the Earth's core. The geomagnetic field is known to show oscillations and variability on a wide range of timescales. These are believed to be the observable consequence of the rich dynamics taking place in the outer core of the Earth, where fluid motions driven mainly by thermochemical convection generate and constantly modify the geomagnetic field. To link the geomagnetic observations with the core flows is an incredibly difficult feat that challenges our technological capabilities and scientific understanding of the structure and dynamics of the Earth's interior. Here I study the possibility of using the inerannual to decadal geomagnetic oscillations to obtain a dynamical picture of the fluid flow and magnetic fields in the Earth's outer core. This is possible thanks to the quasi-geostrophic approximation, originating from the overwhelming importance of Earth's rotation on the core dynamics.