By Lars Mucklejohn Shares in FTSE 250 lender Onesavings Bank (OSB) have dropped as much as 18 per cent after the group flagged that increased competition in the mortgage market and changing customer behaviour would weigh on its margins later this year. The bank reported a statutory pretax profit of £241.3m for the first half of 2024, compared to £76.7m during the same period last year. On an underlying basis, pretax profit grew to £249.9m from £116.6m. It announced a new £50m share buyback to reward investors. OSB’s bottom line was hit by an adverse effective interest rate (EIR) adjustment las…