  Isoscalar (T=0) plus isovector (T=1) pairing hamiltonian in LS-coupling,
which is important for heavy N=Z nuclei, is solvable in terms of a O(8) algebra
for some special values of the mixing parameter that measures the competition
between T=0 and T=1 pairing. The O(8) algebra is generated, amongst others, by
the $S=1,T=0$ and $S=0,T=1$ pair creation and annihilation operators . Shell
model algebras, with only number conserving operators, that are complementary
to the $O(8) \supset O_{ST}(6) \supset O_S(3) \otimes O_T(3)$, $O(8) \supset [
O_S(5) \supset O_S(3) ] \otimes O_T(3)$ and $O(8) \supset [ O_T(5) \supset
O_T(3)] \otimes O_S(3)$ sub-algebras are identified. The problem of
classification of states for a given number of nucleons (called `plethysm'
problem in group theory), for these group chains is solved explicitly for
states with O(8) seniority $v=0, 1, 2, 3$ and 4. Using them, band structures in
isospin space are identified for states with $v=0, 1, 2$ and 3.
