When World War I began, Oskilko was drafted into the Imperial Russian Army, where he had a successful career, reaching the rank of lieutenant colonel.

By the end of 1917 he had returned to his native Volyn in Ukraine, where he participated in the formation of the Ukrainian People's Army.

Oskilko participated in extinguishing the Bolshevik-led Polissya Uprising that was directed against the Directorate of Ukraine and preventing the advancement of the Polish army into Volyn.

On April 12, 1919, a new government, the Council of People's Ministers, was formed, headed by Borys Martos and replacing the Ostapenko cabinet.

Martos proclaimed the creation of a republic of worker's Councils (Soviets) and the intention to conclude a peace treaty with Bolshevik Russia.

On April 20, 1919, numerous socialist parties of Ukraine (sovereigntists, federalists, and others) entrusted Oskilko to deliver a memorandum to Petlyura demanding the immediate resignation of the Martos government.

The participants of the putsch requested the appointment of Yevhen Petrushevych as the provisional president of Ukraine until the convocation of the Constituent Assembly, the transfer of command to general Omelianovych-Pavlenko and the dismissal of Petlyura and Andriy Makarenko from the organization of military affairs, and the creation of a coalition government from the members of both Ukrainian republics.