This deliverable presents the OH-FINE Decision Support System (DSS), internally referred to as the Farm Simulator, accessible at https://www.organicfarmingcompass.eu/dss. The DSS is the principal digital tool of the OH-FINE project and constitutes the first Key Exploitable Result (KER1) as defined in the Grant Agreement (GA 101183127). It is designed to support European farmers, advisors, researchers, and policymakers in making informed, data-driven, and sustainability-aligned decisions regarding the transition to organic farming, the consolidation of organic practices, and the continuous improvement of organic production systems.Crucially, the DSS is not conceived as a standalone calculator or isolated decision tool. Instead, it functions as the central orchestration hub of an integrated suite of decision-support instruments embedded within the OH-FINE platform. This suite includes the Organic Compass (a conversational AI knowledge assistant), the Farmer Needs Radar (a problem-solving and innovation matching engine integrate in the Organic Compass), the Cross-Fertilizer Accelerator (a co-innovation matchmaking tool), the Regional Knowledge Hub portals (providing locally grounded knowledge), and a comprehensive resource library covering technical documents, field trial reports, regulatory guides, and multimedia content.The approach adopted in this deliverable reflects a fundamental design principle: that effective decision support in organic farming cannot be reduced to a single analytical engine. Farmers do not make decisions in a vacuum. They need context, knowledge, training, peer examples, regulatory clarity, economic analysis, and environmental impact assessment, often simultaneously. The OH-FINE DSS addresses this reality by acting as an intelligent entry point that draws on all available platform resources to deliver holistic, personalised guidance.This document describes the complete architecture, functional modules, data infrastructure, user interface design, governance framework, and validation plan for the DSS. It is structured to follow the same approach as Deliverable D4.1 (OH-FINE Platform Architecture), providing the technical detail necessary for implementation, testing, and long-term maintenance.