As per the agreed terms, HomeStreet will pay Orange County Business Bank $11.70 per share.
Orange County Business Bank is a full-service commercial bank with close to $200m in assets and total gross loans of over $128m as of 30 June this year.
It will operate as an office of HomeStreet, following the completion of the acquisition.
The acquisition, which is subject to certain customary closing conditions, is expected to be completed in the fourth quarter of 2015 or early in the first quarter of 2016.
OCBB will secure access to additional resources to better serve clients, including higher loan limits and a broader menu of commercial and consumer loan, deposit, investment and insurance products and services.
HomeStreet president and CEO Mark K. Mason said: "Orange County Business Bank brings to HomeStreet a team of very talented and dedicated bankers who know the community and the unique needs of businesses.
"Customers will continue to receive the high-quality, personalized service they currently enjoy and will now have a wider range of products and services available to them from the team they know and trust. The two banks share a similar philosophy on providing exceptional customer service through a relationship-based approach to banking, all with a strong commitment to serving the local community."
The combined company is expected to have around $5.01bn in assets.
Currently, HomeStreet and OCBB together have a network of 112 retail deposit branches and stand-alone lending centers in eight states.
In 2013, HomeStreet agreed to purchase Fortune and Yakima National banks, under two separate merger agreements.
