A half-commutative
orthogonal Hopf algebra is a Hopf ∗-algebra generated by the self-adjoint coefficients
of an orthogonal matrix corepresentation v = (vij) that half commute in the sense
that abc = cba for any a,b,c ∈{vij}. The first nontrivial such Hopf algebras
were discovered by Banica and Speicher. We propose a general procedure,
based on a crossed product construction, that associates to a self-transpose
compact subgroup G ⊂ Un a half-commutative orthogonal Hopf algebra
𝒜∗(G). It is shown that any half-commutative orthogonal Hopf algebra arises
in this way. The fusion rules of 𝒜∗(G) are expressed in term of those of
G.