On Mac OS X systems (e.g., Yosemite), the security defaults for JAVA are quite restrictive. If you experience a warning dialog saying that Onyx cannot be opened because the identity of the developer cannot be verified, right click on the Onyx file and choose open from the menu. We currently cannot provide digital developer signatures. So please make sure that you have downloaded an original Onyx version from the original website. Reasonable caution is required whenever programs downloaded from the internet are executed on your machine.
Also see this discussion on apple.com: https://discussions.apple.com/thread/6551715