The open cluster NGC 6791 is among the oldest , most massive and metal-rich open clusters in the Galaxy . High-resolution H -band spectra from the Apache Point Observatory Galactic Evolution Experiment ( APOGEE ) of 11 red giants in NGC 6791 are analyzed for their chemical abundances of iron , oxygen , and sodium . The abundances of these three elements are found to be homogeneous ( with abundance dispersions at the level of \sim 0.05 - 0.07 dex ) in these cluster red giants , which span much of the red-giant branch ( T _ { eff } \sim 3500K - 4600K ) , and include two red-clump giants . From the infrared spectra , this cluster is confirmed to be among the most metal-rich clusters in the Galaxy ( < [ Fe/H ] > = 0.34 \pm 0.06 ) , and is found to have a roughly solar value of [ O/Fe ] and slightly enhanced [ Na/Fe ] . Non-LTE calculations for the studied Na I lines in the APOGEE spectral region ( \lambda 16373.86Å and \lambda 16388.85Å ) indicate only small departures from LTE ( \leq 0.04 dex ) for the parameter range and metallicity of the studied stars . The previously reported double population of cluster members with different Na abundances is not found among the studied sample .