Although C(sp2)– and C(sp3)–OMe bonds are arguably the simplest and most atom-economical derivatives in the alcohol series, these linkages have rarely been utilized as coupling partners in the context of metal-catalyzed carbon-heteroatom bond-formation. Among other scenarios, Ni-catalyzed borylation of aryl and benzyl methyl ethers via C(sp2)– and C(sp3)–OMe cleavage is a particularly attractive endeavor, given that the resulting organoboranes are privileged synthons of utmost relevance in organic synthesis and materials science. Intriguingly, the nature of the boron reagent dictates whether functionalization occurs at the C(sp2)– or C(sp3)–OMe fragment. Herein, we report a detailed study aimed at understanding the mechanistic underpinnings of this reaction and the origins of site-selectivity. We conclude that the divergency in site-selectivity arises from a different nickel speciation within the catalytic cycle. Specifically, our investigations reveal that catalytic functionalization at the C(sp2)–OMe bond occurs via a Ni(I) manifold in the presence of B2nep2 whereas a canonical Ni(0)/Ni(II) regime is responsible for the corresponding C(sp3)–OMe bond-cleavage with B2pin2 as coupling partner. These findings focus attention on the critical role that metal speciation might have for dictating site-selectivity in other related cross-coupling reactions.