The purpose of this study is to investigate the effects of OPEC oil production cut announcements on stock returns of specified companies listed on the London Stock Exchange. Two categories are constructed from stocks of companies operating in oil, gas and mining sectors and companies operating in pharmaceutical, industrial engineering and industrial transportation sectors, respectively. The study is based on the theories of EMH and findings of behavioural finance and applies a CAPM model in the context of an event study methodology. Our findings show that in four out of five cases OPEC production cut announcements have significant effects on stocks in the chosen categories around the releaseof a supply cut announcement. The difference between post-announcement CAARs of the constructed categories is significant on one occasion. Organisations and investors can use these findings to better understand the impact of OPEC news announcements on the stock performance of companies in specified sectors.