OB associations are large stellar systems composed of groups of young stars spread over several tens of parsecs. They are the prime sites of star formation in galaxies. More than one stellar cluster and distributed stellar populations are found in such stellar systems. This is a general structural feature. It has been believed that such structural features may contain a clue to the formation process of OB associations because OB associations are young. Before the Gaia undertook its own mission, it was very difficult to obtain homogeneous observational data for various associations distributed over wide sky areas. Furthermore, the selection of members was even more limited. The Gaia mission has now opened a new window to study OB associations in detail as it measures the precise parallaxes and proper-motions of stars in the Galaxy. In this talk, I will present the observational results of several galactic OB associations and discuss their formation process.