Catalyzes the attachment of O-phosphoserine (Sep) to tRNA(Cys). ATP + O-phospho-L-serine + tRNA(Cys) = AMP + diphosphate + O-phospho-L-seryl-tRNA(Cys) Homotetramer. Interacts with SepCysS. Belongs to the class-II aminoacyl-tRNA synthetase family. O-phosphoseryl-tRNA(Cys) synthetase subfamily.