First is important to note that ISO 27001 does not prescribe who the asset owner must be, so organizations are free to define the asset owners as best fit them.
Considering that, company board/managers can be the owners of assets like contractors or employees, but as a good practice, you should consider as the asset owner the first management level with responsibility for protecting and managing the asset, because this will make the decisions about the asset faster and more effective.
For example, if the asset is a server, the owner should be the server's administrator. In the case of contractors and employees, you should consider the asset owner the HR manager
This article will provide you a further explanation about the asset owner: