Tullow Oil's recent pull-back due to investor uncertainty over its exploration-led strategy has created a buying opportunity, according to Societe Generale, which has upgraded its rating for the shares from 'hold' to 'buy'."Short-term catalyst-driven investors should now have moved on from the stock, creating an opportunity for longer-term investors to take advantage of the pull-back to add to their positions. In our view, Tullow remains a first-rate explorer."Investec still has some concerns about margins and costs at High Street department store chain Debenhams but has raised its rating for the stock from 'hold' to 'buy' following the interim results on Thursday.Results were in line with the guidance given at the time of the March profit warning, noted analyst Bethany Hocking, "and the lack of further negative surprises should be taken well".The sharp sell-off in the gold price this week on the back of concerns over demand has created some buying opportunities across the gold-mining sector, according to Tradenext.Ronnie Chopra, Head of Strategy at Tradenext, said that he is a buyer of African Barrick Gold, Petropavlovsk, Kazakhmys, Vedanta, Fresnillo and Ferrexpo "at what could be once-in-a-lifetime low prices". However he did recommend traders "of nervous dispositions" to run tight stop losses.BC