I believe "portfolio strategy" could work, depending what the ownership refers to, of course. If its real estate , it could also be "property portfolio strategy".

owners as in shareholder, board member and executive management?

I looked at your source on their web site. I agree that "owner strategy" does not work. "Corporate strategy" is what is being referred to on the web site's description of services. From Wikipedia at the first reference:

"Corporate strategy refers to the overarching strategy of the diversified firm. Such a corporate strategy answers the questions of "in which businesses should we compete?" and "how does being in these business create synergy and/or add to the competitive advantage of the corporation as a whole?"
Business strategy refers to the aggregated strategies of single business firm or a strategic business unit (SBU) in a diversified corporation.

Note the distinction between "corporate strategy" and "business strategy" that corresponds to "eierstrategi" vs. "selskapsstrategi" in your source.

See also this from the Harvard Business School at the second reference at page 3:

"This article presents a comprehensive framework for value creation in the multibusiness company. It addresses the most fundamental questions of corporate strategy: What businesses should a company be in? How should it coordinate activities across businesses? What role should the corporate office play? How should the corporation measure and control performance?"