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ATP synthase delta subunit is a subunit of bacterial and chloroplast F-ATPase/synthase. It is known as OSCP in mitochondrial ATPase.
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 FO complex with the F1 complex. In mitochondria, the peripheral stalk consists of OSCP, as well as FO components F6, B and D. In bacteria and chloroplasts the peripheral stalks have different subunit compositions: delta and two copies of FO component B , or delta and FO components B and B'.
F-ATPases lacking this subunit generally transport sodium instead of protons. They are proposed to be called N-ATPases, since they seem to form a distinct group that is further apart from usual F-ATPases than A-ATPases are from V-ATPases.
Human delta subunit of ATP synthase is coded by gene ATP5O.