  We used the Immersion GRating Infrared Spectrometer (IGRINS) to determine
fundamental parameters for 61 K- and M-type young stellar objects (YSOs)
located in the Ophiuchus and Upper Scorpius star-forming regions. We employed
synthetic spectra and a Markov chain Monte Carlo approach to fit specific
K-band spectral regions and determine the photospheric temperature ($T_{\rm
eff}$), surface gravity ($\log$ g), magnetic field strength (B), projected
rotational velocity ($v \sin i$), and K-band veiling ($r_{\rm K}$). We
determined B for $\sim$46% of our sample. Stellar parameters were compared to
the results from Taurus-Auriga and the TW Hydrae Association (TWA) presented in
Paper I of this series. We classified all the YSOs in the IGRINS survey with
infrared spectral indices from 2MASS and WISE photometry between 2 and
24~$\mu$m. We found that Class II YSOs typically have lower $\log$ g and $v\sin
i$, similar B, and higher K-band veiling than their Class III counterparts.
Additionally, we determined the stellar parameters for a sample of K and M
field stars also observed with IGRINS. We have identified intrinsic
similarities and differences at different evolutionary stages with our
homogeneous determination of stellar parameters in the IGRINS YSO Survey.
Considering $\log$ g as a proxy for age, we found that the Ophiuchus and Taurus
samples have a similar age. We also find that Upper Scorpius and TWA YSOs have
similar ages, and are more evolved than Ophiuchus/Taurus YSOs.
