Understanding the growth of open access (OA) is important for deciding funder policy, subscription allocation, and infrastructure planning. This study analyses the number of papers available as OA over time. The models includes both OA embargo data and the relative growth rates of different OA types over time, based on the OA status of 70 million journal articles published between 1950 and 2019. The study also looks at article usage data, analyzing the proportion of views to OA articles vs views to articles which are closed access. Signal processing techniques are used to model how these viewership patterns change over time. Viewership data is based on 2.8 million uses of the Unpaywall browser extension in July 2019. We found that Green, Gold, and Hybrid papers receive more views than their Closed or Bronze counterparts, particularly Green papers made available within a year of publication. We also found that the proportion of Green, Gold, and Hybrid articles is growing most quickly. In 2019:- 31% of all journal articles are available as OA. – 52% of article views are to OA articles. Given existing trends, we estimate that by 2025: – 44% of all journal articles will be available as OA. – 70% of article views will be to OA articles. The declining relevance of closed access articles is likely to change the landscape of scholarly communication in the years to come.