The FTSE 250 lender, which includes the Kent Reliance brand, posted a profit of £78.4m, compared with £65.3m for the same period in 2016.
The loan book grew by 10 per cent though costs started to rise, with the bank’s cost to income ratio rising to 28 per cent, up from 26 per cent in the same period last year.
One Savings Bank chief executive Andy Golding said recent changes announced to the tax treatment of buy-to-let investments have made the sector “less attractive” for the amateur.
But he said the focus of his company has been on what he calls the “professional” landlord, and he takes the view this group will benefit from the changes because they will face reduced competition from the part-time landlords.
Mr Golding said: “Our proposition has always been targeted towards the professional landlord, hence these changes have had a positive impact on One Savings Bank.
"Our average market share of new business increased in the first six months of 2017, given our specialism and expertise in lending to limited companies and large portfolio landlords.
"Average loan size in our core businesses is [around] £250,000, demonstrating that our loan book primarily consists of standard family homes and smaller flats, typically subject to sustained high demand.”
The results showed only 0.04 per cent of One Savings Bank’s loan book was in default during the period covered by these results.
Mr. Golding said he expects the bank to grow its loan book by a high-teens percentage in 2017, while maintaining the net interest margin, which is the gap between the interest it pays to depositors and the rate of interest it receives from those to whom it lends.
One Savings Bank's net interest margin increased to 322bps, compared with 309bps for the same period last year, with net interest income up by 18 per cent to £117.1m.
The return on equity achieved by One Savings Bank in the period covered by these results was 28 per cent, marginally down on the 29 per cent from the previous year.