Proceed by induction. Certainly Co(0) = 0, as 0 cannot be written as a sum of positive odd integers. Also, Co(1) = 1, since the single composition of 1 consists of an odd part. Assume the claim is true for n-1 and n-2. The compositions counted by Co(n) included the odd part compositions of n-1 with an additional part 1 included at the end, and the odd-part compositions of n-2 with the last part increased by 2 (which results in another odd part). Since any odd part composition of n ends in either 1 or larger odd number,Co(n-1)+Co(n-2) counts all odd part compositions of n by the induction hypothesis.