Operational Modal Analysis (OMA) is an attractive technology for Structural Health Monitoring (SHM) for wind turbine blades as it covers the entire blade with very limited instrumentation. This work points out some caveats inherent in this technology that must be taken care of for successful implementation. In particular, we discuss how the location of damage affects the detection performance. Additionally, measurements of OMA of MW scale wind turbine generator blades are reported. These recordings show that the measurement uncertainty varies within different mode-types, mode-numbers and external conditions. We present tools to the practitioner with which the best modes for damages of certain types and positions can be selected and blind spots on the blade can be estimated. In R&D, validation of OMA type SHM often means inflicting damages at arbitrary positions on a test blade. The results imply that this needs to be augmented with numerical analysis to ensure proper coverage of the critical details in the blade. Finally, incorporating learning’s from field measurements, we discuss strategies to improve resolution for small damages and bring more transparency regarding the detection limits for this monitoring technique.