  The study of the multiplicity of massive stars gives hints on their formation
processes and their evolutionary paths, which are still not fully understood.
Large separation binaries (>50 milliseconds of arc, mas) can be probed by
adaptive-optics-assisted direct imaging and sparse aperture masking, while
close binaries can be resolved by photometry and spectroscopy. However, optical
long baseline interferometry is mandatory to establish the multiplicity of
Galactic massive stars at the separation gap between 1 and 50 mas. In this
paper, we aim to demonstrate the capability of the new interferometric
instrument MIRC-X, located at the CHARA Array, to study the multiplicity of
O-type stars and therefore probe the full range of separation for more than 120
massive stars (H<7.5 mag). We initiated a pilot survey of bright O-type stars
(H<6.5mag) observable with MIRC-X. We observed 29 O-type stars, including two
systems in average atmospheric conditions around a magnitude of H=7.5 mag. We
systematically reduced the obtained data with the public reduction pipeline of
the instrument. We analyzed the reduced data using the dedicated python
software CANDID to detect companions. Out of these 29 systems, we resolved 19
companions in 17 different systems with angular separations between ~0.5 and 50
mas. This results in a multiplicity fraction fm=17/29=0.59+/-0.09, and an
average number of companions fc=19/29=0.66+/-0.13. Those results are in
agreement with the results of the SMASH+ survey in the Southern Hemisphere.
Thirteen of these companions have been resolved for the first time, including
the companion responsible for the nonthermal emission in Cyg OB2-5 A and the
confirmation of the candidate companion of HD 47129 suggested by SMASH+. A
large survey on more than 120 northern O-type stars (H<7.5) is possible with
MIRC-X and will be fruitful.
