The nitroso group has both a +M and -M effect, but the -M effect is more favorable.
Nitrogen has a lone pair of electrons. However, the lone pair of its monomer form is unfavourable to donate through resonance. Only the dimer form is available for +M effect. However, the dimer form is less stable in a solution. Therefore, the nitroso group is less available to donate electrons.
Oppositely, withdrawing electron density is more favourable: (see the picture on the right).As a result, the nitroso group is a deactivator. However, it has available to donate electron density to the benzene ring during the Wheland intermediate, making it still being an ortho / para director.