Oregon Health & Science University and Legacy Health announced Thursday that they had signed a definitive merger agreement and released some details about the transaction, but they suggested it could be months before they send their plans to regulators for approval.

In the nine months since OHSU and Legacy announced their plans to join forces in August, the two Portland health systems have remained tight-lipped on details of their merger plan, suggesting there were still significant details to be ironed out between the two organizations. Thursday’s announcement indicates any major hangups have been resolved.

OHSU agreed to transfer $350 million, an amount that could be affected by closing costs, to the Legacy Health Foundation, which will be spun off as an independent foundation when the deal closes. OHSU also will spend $1 billion on capital improvements at Legacy Health properties over the decade after the deal is finalized.

Under the agreement, Legacy Health facilities will keep their existing names but will be rebranded as OHSU facilities. For example, “Legacy Emanuel Medical Center” will be renamed “OHSU Emanuel Medical Center.”

The combined health system would be governed by a board that includes appointees from each institution, with the present-day Legacy board appointing six of the 11 members.

OHSU’s president would chair the combined board. The agreement also says that the combined system will keep all Legacy employees who are “in good standing.”

OHSU and Legacy are two of the largest health systems in the state, with 10 hospitals between them, all but one in the Portland area. A combined operation would enjoy the largest share of the metro market.

But that level of consolidation will attract the attention of the federal and state antitrust regulators. The Federal Trade Commission, Oregon’s attorney general and the Oregon Health Authority will review the deal, examining whether it will lead to inflated costs to customers as some critics suspect.

The Oregon Health Authority’s new Health Care Market Oversight program will review the deal. State legislators created the regulatory body in 2021 to review business deals involving health care operations like hospitals. OHSU said the two hospital systems expect to submit their application to the state “in the coming months.”