For those using older versions of OBS or StreamLabs, I recommend instead using the Electron Capture app to assist: https://github.com/steveseguin/electroncapture. Follow the link for instructions and files.
To capture audio on macOS using the Electron Capture app, you'll need a virtual audio cable; something to loopback the audio-output back into the system as an input source. Some software options include:
If your mac is unable to handle OBS and VDO.Ninja, another solution use the cloud to host OBS remotely.
Here is an example of a pay-by-the-hour cloud server you can rent for a few dollars: https://console.cloud.google.com/marketplace/details/nvidia/nvidia-gaming-windows-server-2019 It works great, but takes some time to setup for novices. You can also use Paperspace or AWS Workstations as a remote Windows options; Paperspace is easier to get going with. When picking a VM to use, you'll want a machine with a dozen or more vCPU cores, and/or a system with an Nvidia GPU. A GPU works quite well to accelerate RTMP video encoding and VDO.Ninja video decoding.
While Safari may work with VDO.Ninja, it is generally advised to not use Safari with macOS. The microphone may become muted if the tab is minimized, echo-cancellation doesn't quite work as well as with Chromium-based browsers, video/audio issues are more common, and many of the advanced features offered by VDO.Ninja are not supported on non-Chromium-based Browsers. Consider using the Electron Capture app if adverse to installing or using Chrome, as it is based no Chromium but community created specifically for VDO.Ninja.