The CMA said it would be considering if the transaction would result in “a substantial lessening of competition within any market or markets in the United Kingdom for goods and services”.
The government body is now inviting comment on the deal, with a deadline of 19 June. The CMA will then announce if it needs to take the investigation to a second stage by 30 July.
Charter Court is the parent company of Precise Mortgages, while OSB’s brands include Kent Reliance, Interbay and Prestige Finance.
A spokesperson for Charter Court said: “The CMA assessment of the proposed combination is standard procedure and as expected within the transaction process and timetable.”
The firms announced their intention to merge back in March, with the aim of creating a “leading specialist lender”.
They hoped to complete the deal – which would see OSB pick up all of the issues shares of Charter Court, leaving it with a 55 per cent stake in the new group – by quarter three of this year.
Last year OSB saw its gross lending jump to £3bn, while Charter Court lent £2.8bn. Between them they have a loan book of £15.7bn.