Purpose: The ultimate goal of current open access (OA) initiatives is for library services to use OA resources. This study aimed to assess the infrastructure for OA scholarly information ser-vices by tabulating the number and proportion of OA articles in a literature database. Methods: We measured the absolute numbers and proportions of OA articles at different time points across various disciplines based on the Web of Science (WoS) database. Results: The number (proportion) of available OA articles between 2000 and 2021 in the WoS database was 12 million (32.4%). The number (proportion) of indexed OA articles in 1 year was 0.15 million (14.6%) in 2000 and 1.5 million (48.0%) in 2021. The proportion of OA by subject categories in the cumulative data was the highest in the multidisciplinary category (2000-2021, 79%; 2021, 89%), high in natural sciences (2000-2021, 21%-46%; 2021, 41%-62%) and health and medicine (2000-2021, 37%-40%; 2021, 52%-60%), and low in social sciences and others (2000-2021, 23%-32%; 2021, 36%-44%), engineering (2000-2021, 17%-33%; 2021, 31%-39%) and humanities and arts (2000-2021, 11%-22%; 2021, 28%-38%). Conclusion: Our study confirmed that increasingly many OA research papers have been pub-lished in the last 20 years, and the recent data show considerable promise for better services in the future. The proportions of OA articles differed among scholarly disciplines, and designing library services necessitates several considerations with regard to the customers' demands, avail-able OA resources, and strategic approaches to encourage the use of scholarly OA articles.
