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Ultra-deep radio surveys are an invaluable probe of dust-obscured starformation, but require a clear understanding of the relative contribution fromradio AGN to be used to their fullest potential. We study the composition ofthe $\mu$Jy radio population detected in the Karl G. Jansky Very Large ArrayCOSMOS-XS survey based on a sample of 1540 sources detected at 3 GHz over anarea of $\sim350\text{arcmin}^2$. This ultra-deep survey consists of a singlepointing in the well-studied COSMOS field at both 3 and 10 GHz and reachesRMS-sensitivities of $0.53$ and $0.41\mu$Jy beam$^{-1}$, respectively. We findmulti-wavelength counterparts for $97\%$ of radio sources, based on acombination of near-UV/optical to sub-mm data, and through a stacking analysisat optical/near-infrared wavelengths we further show that the sources lackingsuch counterparts are likely to be high-redshift in nature (typical$z\sim4-5$). Utilizing the multi-wavelength data over COSMOS, we identify AGNthrough a variety of diagnostics and find these to make up $23.2\pm1.3\%$ ofour sample, with the remainder constituting uncontaminated star-forminggalaxies. However, more than half of the AGN exhibit radio emission consistentwith originating from star-formation, with only $8.8\pm0.8\%$ of radio sourcesshowing a clear excess in radio luminosity. At flux densities of $\sim30\mu$Jyat 3 GHz, the fraction of star-formation powered sources reaches $\sim90\%$,and this fraction is consistent with unity at even lower flux densities.Overall, our findings imply that ultra-deep radio surveys such as COSMOS-XSconstitute a highly effective means of obtaining clean samples ofstar-formation powered radio sources.

A Multi-wavelength Analysis of the Faint Radio Sky (COSMOS-XS): the Nature of the Ultra-faint Radio Population