Executing on a bold transformation vision beyond the legacy
OSB Group, a UK-listed specialist mortgage lender, has accelerated its innovation journey by modernising its technology landscape with Mambu, enabling simplified operations, and customer-centric digital savings and mortgages.
As of 31 December 2025
- billion net loan book
- billion retail deposits
- employees
OSB Group
About the customer
OSB Group is a UK-listed specialist mortgage lender, dedicated to supporting underserved segments of the market. They specialise in lending and retail savings through well-known brands, focusing on specific sub-segments, including Buy-to-Let, Residential and Commercial.
The bank’s 6 lending brands, including Rely, Precise, and InterBay, serve a diverse range of customers and needs.
With £25.3 billion in retail deposits and a Common Equity Tier 1 (CET1) ratio of 15.8% as at 31 December 2025, OSB Group continues to be a strong player in the UK’s mortgage and savings sectors.
The challenge
Future-proof tech landscape
To keep pace with evolving market demands and growth plans, OSB Group, a growing specialist lender with a rich history, proactively recognised the necessity of upgrading its systems.
The lender embarked on a comprehensive five-year transformation journey in 2022 to reimagine its vision, develop a strategic roadmap for operational modernisation, build future-proof banking capabilities and deliver a robust, scalable and customer-centric platform.
OSB Group needed a core solution that would facilitate easy integration of new technologies and rapid delivery of customer-centric financial products in a competitive and highly regulated market.
The solution
Modernising into a bank with the soul of a fintech
OSB Group chose Mambu’s cloud banking platform as the core banking solution at the heart of its multi-year transformation journey.
Underpinned by Microsoft Azure, and integrated with nCino and an ecosystem of fintech solutions through a partnership with Publicis Sapient, enables OSB Group to modernise its technology stack.
These advancements are propelling its digital transformation and strengthening the bank’s position as a market leader in specialist lending.
Mambu’s composable banking approach aligned with its goal to break away from monolithic legacy systems and move toward a modular architecture that can easily adapt to evolving customer and regulatory demands.
Through this key partnership, Mambu is instrumental in helping OSB Group streamline its technological infrastructure, enhance operational efficiency, accelerate the development and launch of innovative digital savings, mortgage, and lending products, and substantially reduce time to market.
“This collaboration highlights our commitment to transforming our core banking system to better support specialist lending and savings. It also reflects a wider industry move away from legacy systems towards cloud-native, composable platforms that enable greater flexibility, innovation, and responsiveness in a rapidly evolving regulatory and competitive environment.”
- Matthew Baillie, Group Chief Operating Officer at OSB Group
The result
Modern, customer-centric savings and mortgages
Two years into its partnership with Mambu, OSB Group has already seen a significant uplift in the speed of its operations. The specialist mortgage lender successfully launched new competitive digital savings products through their Kent Reliance brand on the Mambu platform in a matter of months, starting with fixed-rate bonds in late 2024, followed by easy-access accounts in July 2025, with ISA products set to launch in 2026.
The new digital savings products offer simplified onboarding and real-time servicing capabilities.
The new digital savings products offer simplified onboarding and real-time servicing capabilities. Thanks to Mambu's flexible product configuration, OSB Group was able to tailor interest structures and terms more efficiently to market demand. Since its debut, this new offering has quickly grown balances and received overwhelmingly positive customer reviews.
This newly launched offering enhances OSB Group’s savings business and laid the foundation for a major overhaul of their lending platform. With Buy-to-Let mortgages launched in November 2025 through their brand-new Buy-to-Let lender brand Rely, and residential mortgages set for 2026, OSB continues to transform into a modern, agile financial institution delivering innovative, customer-centric products.
“With Mambu, we are not just replacing a system–we’re rethinking how we build, launch, and manage financial products. This positions us to better meet the evolving needs of our customers while maintaining operational efficiency and scalability.”
- Debra Bailey, Group Chief Information Officer at OSB Group
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