During the Japanese occupation, Osa worked in the propaganda section of a Hōkōkai and became a member of the Suishintai, however, he was briefly detained by the Kenpeitai over his connections to an underground resistance movement.

His support of the previous parliamentary system and opposition to President Sukarno's Guided Democracy led to a power struggle within the PNI, between Osa's right-wing and Ali Sastroamidjojo's left-wing.

In the aftermath, Osa was captured and sentenced to jail for 4 years and two months, before being exiled to the Boven-Digoel concentration camp by Dutch authorities.

[2] During his time in Boven-Digoel, Osa became a member of the Rust en Orde Bewaarder (ROB), a security force which protected the streets and villages of the area.

[6] Following the proclamation of independence and the beginning of the Indonesian National Revolution, he served in the newly formed provincial government of the province of West Java.

Through a number of factors, including the skillful manipulation of the congress schedule, and the support of President Sukarno, Ali was re-elected chairman through acclamation.

This resulted in Osa being appointed a deputy chairman, along with Hardi, Mohamad Isa, Ruslan Abdulgani, and I Gusti Gde Subamia.

[9] The defeat of the right-wing Hardi faction in 1963 PNI Congress, led to their growing isolation in the subsequent years, with the left-wing continuing to gradually increase the number of their supporters in both the national and local party councils.

Instead, the day before the meeting, they drafted a letter, which was delivered to all Central Leadership Council members, explaining that their refusal was based on their belief that nothing would be achieved, due to the existing tension in the party.

[14] By mid-October, army commanders and religious groups had backed and cooperated with Osa's faction in eliminating supposed pro-PKI elements from PNI.

[15] While other PNI members such as Iskaq Tjokrohadisurjo, attempted to bring Ali's and Osa's factions together by arranging an emergency party congress with little success.

In the third meeting, on 24 March, Suharto extracted a joint statement from the Osa and Ali factions, in which both groups declared their agreement to hold an extraordinary congress.

Eventually, both groups agreed to the appointment of Osa as General Chairman with full authority to choose a new central leadership council, with the assistance of S. Hadikusumo, a supporter of Ali.

Suharto's actions were based on his view of the PNI, which he saw as a natural vehicle for the expression of the political aspirations of the non-Islamic masses of Java and as an important counter-balance towards the Muslim parties, such as Parmusi, and the Nahdlatul Ulama.

[22] On the night of 14 September 1969, Osa was invited to give a speech at the opening of the 5th Indonesian National Student Movement congress in Salatiga, Central Java.