Purpose: There is increasing research interest in the expansion of the offshore wind energy sector. Recent research shows that operations and maintenance (O&M) account for around 20-35 per cent of the total energy costs in this sector. The purpose of this paper is to provide an overview of O&M issues in the offshore wind energy sector to propose initiatives that can help reduce the cost of energy used by offshore wind farms. Design/methodology/approach: The paper is based on an in-depth literature review and a Delphi study of a panel of 16 experts on O&M. Findings: Consisting primarily of conceptual papers and/or modelling papers, the extant literature identifies several challenges for O&M in the offshore wind energy sector. These challenges can be grouped into four categories: issues related with industry immatureness; distance/water depth; weather window; and policy issues. The Delphi study identified three other major issues that lead to increased O&M costs: too many predefined rules that limit development; lack of coordinated planning of the different services offered at the wind farms; and lack of a common approach on how O&M should be managed strategically. Research limitations/implications: The present study is based only on Danish respondents. Future research needs to include various respondents from different countries to identify country-specific contingencies. Practical implications: The paper provides an overview of the O&M issues in the offshore wind energy sector to prioritize where future resources should be invested and, thus, reduce O&M costs. Originality/value: This is the first paper on O&M issues that bridges both literature studies and industry expert opinions.