Eastgate is sales and marketing director at OSB and has been at the lender for more than six years.
He will join Shawbrook as managing director for commercial mortgages, Shawbrook has confirmed.
A statement from the lender said Eastgate was a highly respected name within the industry.
It added: “With current responsibilities including the development and positioning of all products and the management of OSB’s intermediary relationships, John promises to be a key figure for Shawbrook as it seeks to grow within their chosen markets in support of professional investors, landlords and SMEs across the UK.”
Eastgate’s career includes nine years at Saffron Building Society where he rose to sales and marketing director and 10 years with Barclays.
Shawbrook Commercial Mortgages sales director Emma Cox will continue to lead the firm until Eastgate has completed his notice and contractual commitments.
Eastgate said: “The commercial mortgages division at Shawbrook has a long track record of success and this represents a great opportunity to take an established business on the next phase of its journey.
“I’ve loved my time at OSB and value the relationships and successes I have experienced over the past six years.
“This was the right time to make a change, and I’m looking forward to what I’m sure will be a challenging yet highly rewarding next step.”
Shawbrook CEO Ian Cowie added: “Securing John’s appointment is a fantastic way to kick off the year and I’m delighted that we will be welcoming him to the commercial mortgages business in June.
“His knowledge and expertise across this space will be invaluable in helping us continue our growth across the commercial property market.”
A One Savings Bank spokesman thanked Eastgate for his time at the business and for building it up to more than £9bn of assets.
“We can confirm that John Eastgate will be leaving OneSavings Bank,” he said.
“We wish him all the best in his next challenge.
“Adrian Moloney, who has been our sales director for three years and is an experienced and respected industry figure, continues to lead sales for the bank and we have no immediate plans to recruit a direct replacement for John.”