After being approved by Karavelov, Nikola Obretenov along with Dimitar Gorov of Giurgiu developed an illegal channel for transferring mail, printed materials, and weapons between Bucharest–Giurgiu and Rousse.

Following Lyuben Karavelov's and Angel Kanchev's orders, Nikola Obretenov, Toma Kardzhiev, and Radi Ivanov established the Rousse private revolutionary committee.

They accepted the new statutes of BRCK, re-elected Karavelov as a chairman of the Bucharest committee, and authorized Vasil Levski to form a "Temporary government" in Bulgaria, wherever he would find appropriate.

Near the end of June 1872, Nikola Obretenov transported the printed statutes, the receipts and other documents, as well as Levski's uniform, sabre, and gun through the developed illegal channel, assisted by baba Tonka, Petrana Obretenova, Todorka Mirazchieva, and Natalia Karavelova.

The country would be divided into the following regions: Stoyan Zaimov was given the task to set Tsarigrad "on fire", Hristo Botev had to bring Filip Totyu along from Odessa, and Panayot Hitov was to negotiate with Belgrade.

Until the outburst of the Stara Zagora uprising of 16 September 1875, Nikola Obretenov actively took part in the preparation of the Chervena Voda detachment and the training of population in nearby villages.

There, taking into account the situation, Zaimov and Nikola Obretenov decided that he and Georgi Apostolov should return to Romania to buy weapons, which they could bring to Vratsa with reinforcements, when the uprising burst out.

Nikola Obretenov and Georgi Apostolov, assisted by the Craiova revolutionary committee, bought weapons and ammunitions, and Hristo Botev passionately took up organizing the detachment.

.. Don't you ever seek glory, but honour and justice—it is the most sacred thing on this world.On Monday, 17 May 1876 Nikola Obretenov with the detachment led by Hristo Botev came ashore from the Radetzky steamship on the bank of Kozloduy, where the rebels, kissing their native soil, took their oaths under the flag, crafted by his sister Petrana Obretenova.

After a month of fighting, roaming, and starvation through the grim mountains, on 17 June Nikola Obretenov and his companions Sava Penev, Dimitar Todorov, and Stoyan Lovchaliyata were betrayed by the brothers Pencho and Petar, and a Turkish posse captured them in the "Balyuviya han" inn near Shipkovo, Troyan.

On 2 July 1876 in the St Jeanne d'Arc stronghold, Nikola Obretenov met his brother Angel, who had been in exile for eight years, for being a member of the detachment of Hadzhi Dimitar and Stefan Karadzha.