  Context: The detection of overabundances of $\alpha$-elements and lithium in
the secondary star of a black-hole binary provides important insights about the
formation of a stellar-mass black-hole. $\alpha$-enhancement might
theoretically also be the result of pollution by the nucleosynthesis occurring
during an outburst, or through spallation by the jet. Aims: We study the
abundances, and their possible variations with time, in the secondary star of
the runaway black-hole binary GRO J1655--40, in order to understand their
origin. Methods: We present a detailed comparison between a Keck spectrum
obtained in 1998 found in the literature, archival VLT-UVES data taken in 2004
and new VLT-UVES spectra obtained early 2006. We carefully determine the
equivalent widths of different $\alpha$-elements (Mg, O, Ti, S and Si) with
their associated uncertainty. We use the well-studied comparison star HD 156098
as well as synthetic spectra to match the spectrum of GRO J1655--40 in order to
determine the abundances of these elements. Results: We see no significant
variations of equivalent widths with time. Our fit using HD 156098 reveals that
there is significant overabundance of oxygen in all our spectra, but no
overabundances of any of the other $\alpha$-elements. Finally, we do not detect
the lithium line at 6707 \AA. Conclusions: We show that there is no detected
pollution in GRO J1655--40 after the burst in 2005. Moreover, we argue that
uncertainties in the equivalent widths were previously underestimated by a
factor of $\sim$3. Consequently, our results challenge the existence of general
overabundances of $\alpha$-elements observed in this galactic black-hole
binary, and thus the accepted interpretation that they are of supernova origin.
The physical cause of the overabundance of oxygen remains unclear.
