  The transition from early Of stars to WN type objects is poorly understood.
O-type supergiants with emission lines (OIf+) are considered to be intermediate
between these two classes. The scope of this paper is to investigate the
spectral variability of three Of+ supergiants. We constituted spectral time
series of unprecedented quality for our targets (~ 200 spectra in total),
essentially in the blue domain, covering time-scales from a few hours up to a
few years. Time Variance Spectrum (TVS) and Fourier analyses were performed in
order to characterize their spectral variability. We report on a correlated
significant line profile variability in the prominent He II \lambda 4686 and
H\beta lines most likely related to the strong stellar winds. The variability
pattern is similar for the three stars investigated (HD14947, HD15570 and
HD16691), and the main differences are more quantitative than qualitative.
However, the reported time-scales are somewhat different, and the most striking
variability pattern is reported for HD16691. We did not find any clear evidence
for binarity, and we focus mainly on an interpretation based on a single star
scenario. We show that the behaviour of the three stars investigated in this
study present strong similarities, pointing to a putative common scenario, even
though a few differences should be noted. Our preferred interpretation scheme
is that of Large Scale Corotating Structures modulating the profile of the
lines that are produced in the strong stellar wind.
