ATP synthase delta subunit is a subunit of bacterial and chloroplast ATPase, or OSCP (oligomycin sensitivity conferral protein) in mitochondrial ATPase (note that in mitochondria there is a different delta subunit, IPR001469).
The OSCP/delta subunit appears to be part of the peripheral stalk that holds the F1 complex alpha3beta3 catalytic core stationary against the torque of the rotating central stalk, and links subunit A of the F0 complex with the F1 complex. In mitochondria, the peripheral stalk consists of OSCP, as well as F0 components F6, B and D. In bacteria and chloroplasts the peripheral stalks have different subunit compositions: delta and two copies of F0 component B (bacteria), or delta and F0 components B and B (chloroplasts).
Human delta subunit of ATP synthase is coded by gene ATP5O.