  Based on a set of over 100 medium- to high-resolution optical spectra
collected from 2003 to 2009, we investigate the properties of the O-type star
population in NGC6611 in the core of the Eagle Nebula (M16). Using a much more
extended data set than previously available, we revise the spectral
classification and multiplicity status of the nine O-type stars in our sample.
We confirm two suspected binaries and derive the first SB2 orbital solutions
for two systems. We further report that two other objects are displaying a
composite spectrum, suggesting possible long-period binaries. Our analysis is
supported by a set of Monte-Carlo simulations, allowing us to estimate the
detection biases of our campaign and showing that the latter do not affect our
conclusions. The absolute minimal binary fraction in our sample is f_min=0.44
but could be as high as 0.67 if all the binary candidates are confirmed. As in
NGC6231 (see Paper I), up to 75% of the O star population in NGC6611 are found
in an O+OB system, thus implicitly excluding random pairing from a classical
IMF as a process to describe the companion association in massive binaries. No
statistical difference could be further identified in the binary fraction,
mass-ratio and period distributions between NGC6231 and NGC6611, despite the
difference in age and environment of the two clusters.
