Mac OS X Softwares to Onion Skinning

Pencil2D is a free and open-source 2D animation software for Windows, macOS, and Linux. It offers a straightforward platform for creating traditional hand-drawn animations using vector and raster graphics.

Synfig Studio is a powerful, free, and open-source 2D animation software designed for creating high-quality, professional-looking vector animations. It offers a timeline-based workflow, extensive control over layers and parameters, and support for various animation techniques, making it a robust tool for artists and animators.