The lender has consistently lent 80% of its total in buy to let over the last two years and expects the 3% surcharge to impact amateur landlords planned to continue to target the professional sector.
The lender reported net advances of £460m in the first three months of 2016, accelerated by acquisitions of £131m first and second charge residential mortgage portfolios.
The challenger bank predicts a cost to income ratio of around 26% in 2015.
It said return on equity was strong in the first quarter despite the introduction of the bank surcharge and the group has drawn £576m under the Funding for Lending Scheme with £700m targeted in 2016.
Andy Golding, CEO of OneSavings Bank, said: “As expected, March was a record month as we fulfilled heightened demand to accelerate mortgage completions ahead of Stamp Duty changes. Intermediary focus has now returned to new applications and we are confident in achieving our full year growth targets.”
He added: “We remain confident in the outlook for 2016.”
In March, the lender’s 2015 annual results showed gross mortgage lending figures rose to £1.8bn, up by 20%, from £1.5bn in 2014.
OneSavings Bank’s lending back book consists of £3.1bn of buy-to-let loans, £2bn of residential mortgages and £49.4m of personal loans.
Sales and marketing director John Eastgate said the bank is planning to grow its support for intermediary partners this year.
OneSavings Bank is made up of a number of specialist financial services businesses including Kent Reliance, Interbay Commercial and Prestige Finance.