Witt invariants of quaternionic forms

We describe all Witt invariants of anti-hermitian forms over a quaternion algebra with its canonical involution, and in particular all Witt invariants of orthogonal groups $O(A,\sigma)$ where $(A,\sigma)$ is an central simple algebra with orthogonal involution and $A$ has index $2$. They are combinations of appropriately defined $\lambda$-powers, similarly to the case of quadratic forms, but the module of invariants is no longer free over those operations. The method involves extending the scalars to a generic splitting field of $A$, and controlling the residues of the invariants with respect to valuations coming from closed points in the Severi-Brauer variety.