The Siemens-Schuckert Orenstein & Koppel (normally abbreviated to Siemens O&K) is an underground car formerly used on the Buenos Aires Underground first built by Siemens-Schuckert and Orenstein & Koppel in 1934, 1937 and 1944 with a smaller number of cars built in Argentina during the 1950s.

The Siemens O&K rolling stock made up the entirety of the trains used on the three lines built by the Hispanic-Argentine Company for Public Works and Finances (CHADOPyF) and has since served on every line of the Underground (with the exception of Line B, which uses third rail electrification) from 1934 to 2016, with cars refurbished by the Emepa Group and Alstom continued to function on the network till 2017.

The three lines remained uniform until 1968, when CAF-GEE rolling stock was purchased to bolster numbers in Lines D and E, though these cars shared a similar design and technical specifications to the Siemens cars.

[5][6][7][8] During the 1950s, as part of Juan Perón's five year plans, more of the Siemens cars were built in Argentina by the company Baseler.

[12][13] This also meant that the cars purchased by the CHADOPyF served for the first time on the line first built by their rivals, the Anglo-Argentine Tramways Company (AATC).