FUOr diagnostics in the K band. On the left, a plot of CO 2.30 μm vs. Na I 2.21 μm EWs compares a strong molecule with an atomic feature. On the right, a plot of Na I 2.21 μm vs. Ca I 2.26 μm EWs compares two atomic features. FUOrs are weaker in both atoms compared to standard M-type photospheres at any luminosity, including S-type sources. Na I 2.21 μm in particular increases toward lower temperatures and higher surface gravities, explaining why lower gravity FUOrs are weak in Na I 2.21 μm. CO 2.30 μm strength grows weakly with decreasing temperature but is also strongly inversely proportional to surface gravity, explaining why FUOrs have intermediate CO 2.30 μm strength, generally between dwarfs and giants.