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With about 12 000 OB type stars selected from the LAMOST and Gaia survey, westudy their 3 dimensional velocity distribution over the range ofgalactocentric radius from 6 to 15 kpc in the Galactic disk plane. A clearripple pattern in the radial velocity ($V_R$) map is shown. The median $V_R$reaches $-8$ km s$^{-1}$ at $R\sim9$ kpc, then increases to $\sim0$ km s$^{-1}$at $R\sim12$ kpc, and later declines to below $-10$ km s$^{-1}$ beyond$R\sim13$ kpc. The median azimuthal velocity ($V_\phi$) map shows a similarpattern but has roughly $1/4$ phase difference with the radial velocity.Although the ripple of negative $V_R$ at $\sim9$ kpc extends to about40$^\circ$ in the azimuth angle, it does not align with either the Local or thePerseus spiral arm. Moreover, the farther ripple beyond 13 kpc does not matchthe Outer spiral arm either. This indicates that the non-axisymmetric kinematicfeatures are not induced by perturbations of known spiral structures. Thecentral rotating bar can not lead to such patterns in the outer disk either.External perturbation of a dwarf galaxy or a dark matter sub-halo can inducesuch patterns but requires more evidence from both observations andsimulations. The $V_\phi$ map in the $Z$--$V_Z$ plane of the OB stars is alsoinvestigated. Despite asymmetry to some degree, no spiral pattern is found.This is reasonable since most of the OB stars have ages much younger than 100Myrs, which is smaller than one orbital period around the Galactic center.

Ripple patterns in in-plane velocities of OB stars from LAMOST and Gaia