As the effects of climate change have been negatively impacting the environment, several industries have started to work towards minimizing them. The construction industry is responsible for 40% of the global carbon emissions and many companies are currently working on net-zero and other sustainability goals. As several sustainability practices are implemented in sustainable construction projects, complexity, as well as opportunities, can arise in the front-end phase of projects. The positive impact that complexity can have on projects, as well as the link between complexity and opportunity are overlooked in the literature. The main objective of the study is to fill these two knowledge gaps by developing a framework which illustrates how complexity can generate opportunities. This is achieved by using three main qualitative research methods: literature review, case study analysis, and expert evaluation, in the context of sustainable construction fit-out projects in the private construction sector, with the focus on the front-end phase of projects. The current study shows that complexity in sustainable construction projects is different from general construction projects, due to factors such as the project sustainability scope and goals, sustainability strategies that are implemented, and required team and knowledge. By comparing four projects in the Netherlands, Ireland, and the UK, it is found that the most common elements of complexity in sustainable construction projects are fixed project schedule, project team’s lack of expertise in implementing sustainability, difficulties in implementing sustainable project management, and stakeholders’ lack of understanding of sustainability. The link between complexity and opportunity is illustrated through the connection between complexity and risk, while the way to generate opportunities from complexities is suggested by using management strategies to deal with complexity. The main management strategies found in the study are analysed in terms of the control and interaction allowed in each of them. Some of the strategies are the clear understanding of sustainability requirements, engaging the relevant stakeholders early upfront, and analysing and suggesting different scenarios and methods. Some of the most common opportunities in sustainable construction projects are related to sustainability management, work processes, lifestyle, training and education, costs, and quality. The CMO opportunity-identifying framework is developed, which shows that project members can generate opportunities from complexities in sustainable construction projects by using combinations of management strategies based on control and interaction. A few recommendations for project members in the industry are to be aware of complexity and to actively look for opportunities. Clients are advised to clearly translate sustainability into project requirements and to allow flexibility in the project schedule and budget. Further research can be conducted to test the CMO framework and analyse complexity, opportunities, and management strategies in different types of projects, countries, and by considering other stakeholders.