This work reports an on-chip heater structure fabricated in the Front End of Line (FEOL) on a versatile ring-oscillator (RO) array utilized to conduct statistical characterization of on-chip annealing of statically, i.e., Bias Temperature Instabilities (BTI), and dynamically, i.e., BTI-Hot Carrier Degradation (HCD), stressed ROs. The heater allows to reach local temperatures up to T = +300 • C in milliseconds. Accurate thermal modelling of the heater structure has been conducted demonstrating that the heat is homogenously distributed across the array area, resulting in a valuable tool for precise on-chip circuit anneal.Exploiting the on-chip heater features, sequential annealing tests with increasing duration have been conducted after accelerated degradation of RO circuits. Moreover, we report detailed relaxation maps that allow a thorough insight into the annealing behaviour of our circuits vs temperature and time. This can potentially be used to anneal ICs "on-line" to prolong their useful lifetime towards a more sustainable use.