Oksana became politically active during her studies of philosophy in Khmelnytskyi. There, she read socialist, Marxist, and feminist thinkers whose work radically transformed her worldview, shifting it from religious orthodoxy toward anarchism. At the Centre of Youth Perspectives in Khmelnytskyi, Ukraine, Oksana encountered Anna Hutsol and Sasha Shevchenko. In 2008, the three of them co-founded FEMEN. Oksana was responsible for the artistic aspects of the project, developing the visual and performative language of the movement: a woman with bare breasts and open arms — an image inspired by Eugène Delacroix's painting Liberty Leading the People — with a flower wreath on her head, an element referencing Ukrainian identity.

In many ways, Oksana foresaw the danger of the Russian authoritarian regime for Ukraine and Europe, and warned of war before its onset. In 2012, FEMEN activists staged a protest during Vladimir Putin's visit to Brussels, where the Russia–EU summit was taking place. The activists attempted to "drive out" the Russian president from the Belgian capital, warning the European Union of the catastrophic consequences for democracy resulting from close ties with Kremlin officials. The protest was held under the slogans: "Putin — Euro-collapse!" and "The Apocalypse of Democracy!"

The image of radical activism created by FEMEN gained popularity not only in Ukraine but worldwide. The success of FEMEN lay in the striking visual identity of their public performances, developed by Oksana, as well as in their strong political slogans. "How can such a beautiful woman — one you would normally see in a beauty salon — be a feminist? I used to see her in pornography, in newspapers, in my bed, or in the kitchen. I am not used to seeing this object protest," explained Anna Hutsol, co-founder of FEMEN, in response to the frequently asked question: "Why use the naked body?" That contradiction became FEMEN's driving force, and the first — at that time not fully conscious — continuation of Oksana's artistic practice, taking place on the street.

"Since 2008, we have been fighting against this system, and for the law, with our means, in the name of personal freedom but also in the name of humanity, to defend the rights of women and their place in society. These aren't big words: I need to be able to lead by example if I believe in what I'm doing. We thus protested against Putin, against the Belarusian dictator Lukashenko. I was arrested, covered in feathers and dyed green, beaten, tortured. I thought the end was near. I am psychologically ready to be disfigured or killed, even if I, like everyone, experience fear in everyday life. The weeks of preparation that precede our actions sometimes give rise to apprehension. But this leaves room, in resolute action, for a very particular feeling which is based neither on fear, nor on shame, nor even on courage. There is a feeling of great strength supported by the thought that our actions are right. If I am ready to give my life for my ideas? But I already gave it! I gave it, driven by a conviction: true art is revolution." — Oksana Shachko