The Kelin-Kaluza version of the combined Einstein-Yang-Mills system is based on a larger-dimension manifold, admitting an O(4+N) symmetry, which undergoes compactification. The theory provides a unified treatment of all gauge fields and its action consists of a single invariant Lagrangian and, separately, a single gauge-fixing term. The authors show that both Lagrangians can be combined within a larger framework, admitting an OSp(4+N/2) supersymmetry, which undergoes appropriate dimensional reduction; here the extra graded dimensions are associated with the fictitious fields needed for consistent quantisation and unitarity.