Kinematic properties of the O stars in the vicinity of NGC 6611. Left: position and relative proper motion of the O stars near NGC 6611 overlapped on the DSS2 B, R and I colour image. The O stars shown in blue and cyan have velocities comparable or greater than the escape velocity and move away from the centre of NGC 6611. The O stars shown in magenta do not have this clear transverse motion away from the centre of NGC 6611, but could have had dynamical interactions in the past (see Appendix B. The O stars in red have no evidence for dynamical interactions in the past. The angular distance equivalent to 3 pc is given for a distance of 1.70 kpc. We highlight the centre of NGC 6611 with the dashed white circle (r = 0.05 deg). Right: relative (systemic) radial velocity Δv_R as a function of the absolute relative transverse velocity |Δv_T| for the O stars in NGC 6611, coloured similarly to the left panel. The O stars for which we have not used the Gaia astrometry (due to ruwe > 1.4) and thus have unknown |Δv_T| are shown on the left. The uncertainty on the radial velocity of NGC 6611 determined from the B0 to B4 stars is shown with the horizontal black shaded region. The median outward velocity determined by Kuhn et al. (2019) for NGC 6611 is shown with the vertical black shaded region.