Why is nitrosobenzene electrophilic on O in organocatalytic oxygenations?
Using proline as the catalyst provides enantioselectivity to the process, but also allows N vs O regioselectivity to be controlled (though not always perfectly).
Houk (and subsequently others) applied computational methods to the reaction in an attempt to understand why the nitrosobenzene was acting as an electrophile at oxygen rather than nitrogen, as would traditionally be expected (and as is often the case when no proline is used). The computational studies showed a clear preference for the transition state in which the basic nitrogen of nitrosobenzene was able to interact with the acidic proton of the proline:

Proline catalysis occurs via a proline enamine attack on the oxygen of the nitrosobenzene monomer with a simultaneous proton transfer from the carboxylic acid. Computational evidence shows that the observed heteroatom selectivity materializes through the difference in basicities of the oxygen and the nitrogen atoms.
Source: Cheong, P. H.-Y.; Houk, K. N. J. Am. Chem. Soc. 2004, 126 (43), 13912–13913. DOI: 10.1021/ja0464746.

This study further indicated that the transition state for attack at N (which you drew) is disfavoured by 2.5 kcal/mol:

Ref: Enantioselective Organocatalysis, 2007, Dalco (Wiley)