Some of my research is carried out in the area of Operations Research. This term is actually the American version. The English version is Operational Research and (although being biased), I believe that this is the correct version, bearing in mind that Operational Research was founded in England (around the time of the Second World War).
You will find the distinction between these two terms no more apparent than in the naming of academic journals. The oldest OR journal in the world is called Journal of the Operational Research Society (JORS). Another journal is called European Journal of Operational Research (EJOR) .
The journals published in America have titles such as Operations Research.