I will review a new approach to constructing interacting (Euclidean) QFTs in 4 dimensions via the operator product expansion (OPE), and a number of important results that have been proven in the last years using the Polchinski-Kopper flow equation framework, such as existence, convergence and factorisation properties of the OPE in a perturbative sense for non-gauge theories. Lastly, I present on-going work on the generalisation to gauge theories.