  We present a study aimed at clarifying the birthplace for 43 massive O-type
field stars. In this first paper we present the observational part: a search
for stellar clusters near the target stars. We derive stellar density maps at
two different resolving scales, viz. 0.25pc and 1.0pc from NTT and TNG imaging
and the 2MASS catalogue. These scales are typical for cluster sizes. The main
result is that the large majority of the O-type field population are isolated
stars: only 12% (5 out of 43) of the O-type field stars is found to harbour a
small-scale stellar cluster. We review the literature and aim at characterizing
the stellar field of each O-type field star with the emphasis on star formation
and the presence of known young stellar clusters. An analysis of the result of
this paper and a discussion of the O-type field population as products of a
dynamical ejection event is presented in an accompanying paper.
