Planning large-scale deployment of offshore wind requires coordinated assessment of future electricity demand, transmission infrastructure, electricity market development, and system operations. These elements are often evaluated independently, resulting in inconsistent assumptions and suboptimal infrastructure planning under uncertain future demand. This paper presents an integrated planning framework developed through the Atlantic Canada Offshore Wind Grid Integration and Transmission Study. The framework links electricity market modelling, high-resolution offshore wind resource assessment, geospatial analyses, transmission expansion and electricity system modelling within a single iterative workflow. Future electricity demand scenarios, including domestic electrification, electricity exports, and clean fuels production, are used to define offshore wind deployment pathways, which are subsequently evaluated for transmission requirements and operational performance. Application of the framework demonstrates that Atlantic Canada's technical offshore wind resource substantially exceeds projected deployment requirements. Instead, future deployment is primarily influenced by export market demand, transmission capacity, project economics, and regional coordination rather than resource availability. The study identifies transmission investment pathways to support a range of offshore wind development scenarios and demonstrates the value of integrated, demand-driven planning for evaluating offshore wind deployment under demand uncertainty. While developed for Atlantic Canada, the methodology is transferable to other emerging offshore wind jurisdictions.