Here’s a USB oscilloscope project from a few years ... Quick, small, and useful; a schematic, board layout, firmware, and PC software sources are all available for download.

Utilize analog and digital signal processing techniques in building a low-cost high-precision digital oscilloscope for high school use.

He wanted to improve on the design though, making a smaller unit with a greatly reduced bill-of-materials and with a more streamlined design, so he came up with this STM32-based oscilloscope.