To restrict IAM actions to only administrator roles across all AWS accounts in an organization, the most operationally efficient solution is to create a Service Control Policy (SCP) that allows IAM actions exclusively for administrator roles and apply this SCP to the root Organizational Unit (OU) of AWS Organizations. This method ensures a centralized governance mechanism that uniformly applies the policy across all accounts, thereby minimizing the need for individual account-level configurations and reducing operational complexity.

AWS Documentation on AWS Organizations and Service Control Policies offers comprehensive information on creating and managing SCPs for organizational-wide policy enforcement. This approach aligns with AWS best practices for managing permissions and ensuring secure and compliant account configurations within an AWS Organization.