  A bounded operator $T$ on a separable, complex Hilbert space is said to be
odd symmetric if $I^*T^tI=T$ where $I$ is a real unitary satisfying $I^2=-1$
and $T^t$ denotes the transpose of $T$. It is proved that such an operator can
always be factorized as $T=I^*A^tIA$ with some operator $A$. This generalizes a
result of Hua and Siegel for matrices. As application it is proved that the set
of odd symmetric Fredholm operators has two connected components labelled by a
$Z_2$-index given by the parity of the dimension of the kernel of $T$. This
recovers a result of Atiyah and Singer. Two examples of $Z_2$-valued index
theorems are provided, one being a version of the Noether-Gohberg-Krein theorem
with symmetries and the other an application to topological insulators.
