From the OUP website

For a simple guide to preparing figure files, please see our short guidelines. These guidelines cover recommended file formats, resolution, sizing, and colour management options, to help you achieve the best outcomes online and in print. They should be read in conjunction with any specific guidelines provided on the website of your chosen journal.

Please also consider accessibility when designing your figure, so that your images can be easily understood by colour-blind and visually-impaired readers. Guidelines for preparing different image-types, including recommendations for colour palettes, colour contrast, image layout, and text accessibility.