Extended green objects (EGOs) are a class of high-mass protostars characterised by active outflows seen in Spitzer data. The launching mechanism of outflows in massive embedded objects remains one of the key puzzles of massive star formation theory as it is thought to reflect characteristics of the accretion process. A sample of EGOs exhibiting OH masers were identified by VLA observations, raising the possibility of probing the magnetic fields at the launching region. This contribution introduces new high-resolution full-Stokes EVN observations of OH masers in EGOs targeting the launching region of outflows in high-mass protostars.