We discuss how to compute scattering waveforms for the
classical two-body problem purely from on-shell five-point amplitudes,
generalizing the KMOC (in-in) approach to radiative observables.
Restricting to the case of two Kerr black-holes, we derive the
time-domain waveform at leading order in the Post-Minkowskian expansion
and at quartic order in the spin by using analyticity arguments to
bypass the conventional observable phase-space integration. The result
is expressed only in terms of the three-point amplitude and the
four-point Compton amplitude, which makes it easier to generalize for
the case of generic spinning bodies (i.e. with tidal effects). Finally,
I will show how we can analytically continue the waveform from
hyperbolic to (eccentric) bound orbits.