H.O.S Cokroaminoto is known as the founder of the Islamic Union (SI). It was at Cokroaminoto's house that Soekarno became acquainted with Sarekat Islam (SI) leaders such as Haji Agus Salim and Abdul Muis.
Soekarno is also familiar with Muso, Alimin, Darsono and Semaun who later became known as leftist and Kartosuwiryo who later founded Darul Islam and led the rebellion against Sukarno, although in the end Sukarno himself who signed the death sentence execution of Kartosuwiryo who became his friend when he was young .
They live together in H.O.S Cokroaminoto house to gain knowledge and learn to organize through Sarekat Islam (SI). Here the nationalist spirit of the nation of Indonesia becomes very big. Soekarno also took part in a youth organization in 1918 named Tri Koro Darmo which later changed its name to Jong Java. Sukarno was even active as a writer in a daily newspaper called Oetoesan Indies run by Cokroaminoto.
At Cokroaminoto's house, young Soekarno began to study politics and also learn to make speeches even though he tends to do himself in front of a mirror in his room. At his school Hoogere Burger School or HBS, Soekarno got a lot of knowledge
In 1921 after graduating from Hoogere Burger School or HBS, the young Soekarno then moved to Bandung and stayed at Haji Sanusi's house. Soekarno then became familiar with Douwes Dekker, Tjiptomangunkusumo, and Ki Hajar Dewantara.
Soekarno then went to Technische Hoogeschool (THS) majoring in civil engineering. Technische Hoogeschool (THS) will be transformed into ITB (Institut Teknologi Bandung) as it is now. In the same year of 1921, Soekarno married Siti Oetari the eldest son of H.O.S Cokroaminoto. Soekarno quit college after two months of entering THS but in 1922 he signed up again and then started college and then graduated on May 25, 1926 with the title of Ir (Engineer).
After graduating from THS, Soekarno established an Engineer Bureau in 1926 with Ir. Anwari who worked on design and building design. He also works with Ir. Rooseno designed and built houses.
While in Bandung, Soekarno founded Algemeene Studie Club (ASC) which later became the forerunner of the Indonesian National Party which was established on July 4, 1927. Here Sukarno then began to practice the teachings of Marhaenism. The objective of the formation of the National Party of Indonesia is for the Indonesian nation to be independent and independent of the Dutch colony.