  The T2K experiment presents new measurements of neutrino oscillation
parameters using $19.7(16.3)\times10^{20}$ protons on target (POT) in
(anti-)neutrino mode at the far detector (FD). Compared to the previous
analysis, an additional $4.7\times10^{20}$ POT neutrino data was collected at
the FD. Significant improvements were made to the analysis methodology, with
the near-detector analysis introducing new selections and using more than
double the data. Additionally, this is the first T2K oscillation analysis to
use NA61/SHINE data on a replica of the T2K target to tune the neutrino flux
model, and the neutrino interaction model was improved to include new nuclear
effects and calculations. Frequentist and Bayesian analyses are presented,
including results on $\sin^2\theta_{13}$ and the impact of priors on the
$\delta_\mathrm{CP}$ measurement. Both analyses prefer the normal mass ordering
and upper octant of $\sin^2\theta_{23}$ with a nearly maximally CP-violating
phase. Assuming the normal ordering and using the constraint on
$\sin^2\theta_{13}$ from reactors, $\sin^2\theta_{23}=0.561^{+0.021}_{-0.032}$
using Feldman--Cousins corrected intervals, and
$\Delta{}m^2_{32}=2.494_{-0.058}^{+0.041}\times10^{-3}~\mathrm{eV^2}$ using
constant $\Delta\chi^{2}$ intervals. The CP-violating phase is constrained to
$\delta_\mathrm{CP}=-1.97_{-0.70}^{+0.97}$ using Feldman--Cousins corrected
intervals, and $\delta_\mathrm{CP}=0,\pi$ is excluded at more than 90%
confidence level. A Jarlskog invariant of zero is excluded at more than
$2\sigma$ credible level using a flat prior in $\delta_\mathrm{CP}$, and just
below $2\sigma$ using a flat prior in $\sin\delta_\mathrm{CP}$. When the
external constraint on $\sin^2\theta_{13}$ is removed,
$\sin^2\theta_{13}=28.0^{+2.8}_{-6.5}\times10^{-3}$, in agreement with
measurements from reactor experiments. These results are consistent with
previous T2K analyses.
