Provided are ligand compounds containing two or more diphosphinoamine functional groups, where the two or more diphosphinoamine functional groups are linked by 4 carbon atoms, a catalyst system including the ligand compounds for olefin oligomerization, and a method for olefin oligomerization using the same. The catalyst system for olefin oligomerization according to the present invention has excellent catalytic activity, and yet, exhibits high selectivity to 1-hexene and 1-octene, thus enabling efficient preparation of alpha-olefin.