Complex sentence: It had its own nationwide league called Oberliga which existed throughout the whole GDR history and since 1954 consisted of 14 teams, whose two worst-placed were relegated to the Liga, which was introduced in 1950.
Split into simpler sentences: It had its own nationwide league called Oberliga which existed throughout the whole GDR history. Since 1954 the "DDR-Oberliga" consisted of 14 teams; the two worst-placed teams were relegated to the Liga, which was introduced in 1950.