SNO+ Collaboration reports its second spectral analysis of reactor antineutrino oscillation using 286 ton-yr of new data. The measured energies of reactor antineutrino candidates were fitted to obtain the second-most precise determination of the neutrino mass-squared difference Δm_{21}^{2}=(7.96_{-0.42}^{+0.48})×10^{-5} eV^{2}. Constraining Δm_{21}^{2} and sin^{2}θ_{12} with measurements from long-baseline reactor antineutrino and solar neutrino experiments yields Δm_{21}^{2}=(7.58_{-0.17}^{+0.18})×10^{-5} eV^{2} and sin^{2}θ_{12}=0.308±0.013. This fit also yields a first measurement of the flux of geoneutrinos in the Western Hemisphere, with 73_{-43}^{+47} TNU at SNO+.