Oscar Marcelo Alemán (February 20, 1909 – October 14, 1980) was an Argentine jazz guitarist, singer, and dancer.

Alemán was born in Machagai, Chaco Province, in northern Argentina. He was the fourth child of seven born to pianist Marcela Pereira, a native Argentine, and Jorge Alemán Morales, who played guitar in a folk quartet with his children Carlos, Juan, and Jorgelina.

At the age of six, Alemán joined the family ensemble, the Moreira Sextet, and played the cavaquinho, a chordophone related to the ukulele, before taking up the guitar. The group travelled to Buenos Aires to perform at the Parque Japonés, Nuevo Theater, and at the Luna Park. Later they toured in Brazil.

Alemán was orphaned at age of ten when his mother died and his father committed suicide. He sustained himself by working sporadically as a dancer and musician on the streets of Santos, Brazil. When he saved enough money, he bought a guitar and started to play professionally at party venues in a duo called Los Lobos with his friend, Brazilian guitarist Gastón Bueno Lobo. The duo moved to Buenos Aires in 1925 to work under contract for the comedian Pablo Palitos. In Buenos Aires, they formed a trio with violinist Elvino Vardaro. They added tango to their repertoire and recorded with Agustín Magaldi. They later played with Carlos Gardel and Enrique Santos Discépolo.

In 1929 Los Lobos and dancer Harry Fleming travelled to Europe. After the tour, Alemán stayed in Madrid to play as a soloist. In the 1930s he discovered American jazz through the music of Eddie Lang and Joe Venuti. He then moved to Paris, where he was hired by Josephine Baker to lead her band, the Baker Boys, at the Cafe de Paris, providing him an opportunity to play regularly with American musicians who would come to see Baker and perform with her band. In Paris he met Django Reinhardt, for whom he would sometimes substitute.