Transcranial alternating current stimulation (tACS) is a widespread approach used to understand the causal role of brain oscillations in brain function and behaviour. However, effects of tACS are known to depend on the brain state during stimulation. To reduce variability of stimulation effects, it would be necessary to adapt tACS to ongoing brain oscillations during stimulation. However, electric artifacts in simultaneously recorded electro- and magnetoencephalography (EEG/MEG) have impeded assessment of brain oscillations targeted by tACS. Recently, optically-pumped magnetometry (OPM) was developed, enabling room-temperature MEG recordings directly from the scalp, with a substantially improved signal-to-noise ratio and higher spatial resolution. In this talk, I will show how OPM-MEG can become a useful tool to understand immediate effects of tACS by allowing us to recover evoked alpha and gamma band brain oscillations. This also paves the way for a closed-loop neuromodulation system where tACS is adapted to ongoing brain states (i.e., phase of ongoing oscillation and connectivity measures) that were previously harder to recover precisely due to insufficient signal-to-noise ratio and spatial resolution. Finally, some potential clinical applications of methods mentioned are discussed.