We study the motions inside 28 OB-associations with the use of Gaia DR2 proper motions .
The average velocity dispersion calculated for 28 OB-associations including more than 20 stars with Gaia DR2 proper motion is \sigma _ { v } = 4.5 km s ^ { -1 } .
The median virial and stellar masses of OB-associations are M _ { vir } = 8.9 \times 10 ^ { 5 } and M _ { st } = 8.1 \times 10 ^ { 3 } M _ { \odot } , respectively .
The median star-formation efficiency in parent giant molecular clouds appears to be \epsilon = 1.2 per cent . Gaia DR2 proper motions confirm the expansion in the Per OB1 , Car OB1 and Sgr OB1 associations found earlier with Gaia DR1 data .
We also detect the expansion in Gem OB1 , Ori OB1 and Sco OB1 associations which became possible for the first time now when analyzed with Gaia DR2 proper motions .
The analysis of the distribution of OB-stars in the Per OB1 association shows the presence of a shell-like structure with the radius of 40 pc .
Probably , the expansion of the Per OB1 association started with the velocity greater than the present-day expansion velocity equal to 5.0 \pm 1.7 km s ^ { -1 } .