  We assess the capabilities of magnetic Resonant Inelastic X-ray Scattering
(RIXS) at the O $K$ edge in undoped cuprates by taking La_{2}CuO_{4} as a
benchmark case, based on a series of RIXS measurements that we present here. By
combining the experimental results with basic theory we point out the
fingerprints of bimagnon in the O $K$ edge RIXS spectra. These are a dominant
peak around 450 meV, the almost complete absence of dispersion both with $\pi$
and $\sigma$ polarization and the almost constant intensity vs. the transferred
momentum with $\sigma$ polarization. This behavior is quite different from Cu
$L_3$ edge RIXS giving a strongly dispersing bimagnon tending to zero at the
center of the Brillouin zone. This is clearly shown by RIXS measurements at the
Cu $L_3$ edge that we present. The Cu $L_3$ bimagnon spectra and those at Cu
$K$ edge - both from the literature and from our data - however, have the same
shape. These similarities and differences are understood in terms of different
sampling of the bimagnon continuum. This panorama points out the unique
possibilities offered by O $K$ RIXS in the study of magnetic excitations in
cuprates near the center of the BZ.
