The challenger banks revealed on Monday that they were in advanced talks about a possible merger.
In a joint statement on Thursday, the banks repeated that they believe the deal has a “strong strategic rationale” and would create a “specialist lender in the UK with greater scale and resources to deploy on growth opportunities”.
OSB chief executive Andy Golding would become the boss of the merged company and Charter Court chairman Malcolm Williamson will take the same role at the enlarged group.
Alongside the announcement, the banks both released their 2018 full year results.
OSB said underlying pre-tax profit gained 15% to £193.6mln last year as the loan book rose 23% to £9bn.
However, the net interest margin (NIM) – the difference between interest earned and the amount paid out on deposits – fell to 3.04% from 3.16% last year amid a tough mortgages market.
Charter Court reported a 41.6% increase in pre-tax profit to £158.2mln last year and a 24% rise in the loan book to £6.7bn while its NIM dropped to 3.08% from 3.19%.