Background: The number of approved biosimilars in the United States has increased exponentially in recent years. Within the oncology market, there are currently 14 approved biosimilars, with 11 launched since the start of 2019. The emergence of new oncology biosimilars provides a tool for success in value-based care, and has the potential to lower costs to patients and providers and expand access to care. Methods: Analysis of biosimilar prescribing behavior was performed using 2020 prescription data (Wolters Kluwer, n = 130,836), sales data (IQVIA), and dosage data for patients taking bevacizumab, trastuzumab, rituximab, or an FDA-approved biosimilar for these products (FDA Purple Book), between 2019-2021 (ION Solutions, n = 69,884). Drugs not directly interchangeable with the bevacizumab, trastuzumab or rituximab reference products according to NCCN guidelines were excluded. Results: Biosimilar products are currently available for bevacizumab (2 biosimilars), rituximab (3 biosimilars), and trastuzumab (5 biosimilars). We found that in 2020, 8.2% of new prescriptions for any of these three reference products were for a biosimilar. An analysis of real-world drug administration data revealed that in the 3 months following the 2019 launch of trastuzumab’s first biosimilar (trastuzumab-anns), 7.3% of initiating line 1 patients were prescribed the biosimilar over the reference product. During the same period in 2020, when a total of 5 trastuzumab biosimilars were available, 80.5% of initiating line 1 trastuzumab patients began treatment on a biosimilar, suggesting rapid uptake by providers. However, this differed by product, with the initial uptake for the first rituximab biosimilar (rituximab-pvvr), at only 2.3%. Uptake also occurred within treatment lines, with 11.1% of all patients (bevacizumab: 11.3%, trastuzumab: 14.1%, rituximab: 7.9%) switching from a reference product to a biosimilar during treatment. Uptake was particularly rapid for trastuzumab biosimilars: among patients on trastuzumab at the time of its first biosimilar launch, 18.2% switched to trastuzumab-anns in the first 90 days post-launch. Biosimilars launched at significantly lower prices than their reference products, with cost per prescription at -42.0%, -29.9% and -89.5% relative to the reference product for trastuzumab, rituximab and bevacizumab, respectively. However, biosimilar launches had little impact on reference product pricing, with 2019-2020 year-over-year (YOY) differences in price per prescription close to the YOY averages in previous years (2015-2019) for all three reference products. Conclusions: We conclude that uptake of biosimilars among oncology providers between 2019-2020 was rapid, although the extent of biosimilar prescribing varied among products. Biosimilars offered greatly reduced costs to providers, although reference product prices remained stable despite increased biosimilar competition.