  We present an {\it ab initio}-based theoretical framework which elucidates
the origin of the spin-orbit torque (SOT) in Normal-Metal(NM)/Ferromagnet(FM)
heterostructures. The SOT is decomposed into two contributions, namely, {\it
spin-Hall} and the {\it spin-orbital} components. We find that {\it (i)} the
Field-Like (FL) SOT is dominated by the spin-orbital component and {\it (ii)}
both components contribute to the damping-like torque with comparable magnitude
in the limit of thick Pt film. The contribution of the spin-orbital component
to the DL-SOT is present only for NMs with strong SOC coupling strength. We
demonstrate that the FL-SOT can be expressed in terms of the non-equilibrium
spin-resolved orbital moment accumulation. The calculations reveal that the
experimentally reported oxygen-induced sign-reversal of the FL-SOT in Pt/Co
bilayers is due to the significant reduction of the majority-spin orbital
moment accumulation on the interfacial NM atoms.
