Complex sentence: In 1929 he took part in the first congress of Ukrainian nationalists in Vienna where the Organization of Ukrainian Nationalists (OUN) was born with Konovalets as its leader.
Split into simpler sentences: In 1929 he took part in the first congress of Ukrainian nationalists in Vienna. During the congress it was decided to form the Organization of Ukrainian Nationalists (OUN) and Konovalets was elected as its leader.