We consider an arbitrary o-minimal structure M and a definably connected definable group G. The main theorem provides definable real closed fields R 1 , . . . , R k such that G/Z(G) is definably isomorphic to a direct product of definable subgroups of GLn 1 (R 1 ), . . . , GLn k (R k ), where Z(G) denotes the center of G. From this we derive a Levi decomposition for G, and show that [G, G]Z(G)/Z(G) is definable and definably isomorphic to a direct product of semialgebraic linear groups over R 1 , . . . , R k .