Network operators with large-scale optical transport networks have traditionally relied on a single optical transport supplier to provide end-to-end optical systems within each domain of their network. While integrated optical systems have demonstrated advantages in maximizing performance and minimizing operational complexity, there is renewed interest in optical system disaggregation, especially in open line systems. This white paper examines the drivers and requirements for an open line system that will meet the most advanced needs of network operators today while offering future-proofing for tomorrow.