1. Install OBS

Go to https://obsproject.com/download and click the penguin to see the Linux installation instructions.

Ubuntu Instructions (18.04+)

Run these commands to install OBS Studio on Ubuntu via the official PPA:

sudo add-apt-repository ppa:obsproject/obs-studio
sudo apt update
sudo apt install ffmpeg obs-studio

2. Install the v4l2loopback kernel module for the virtual camera created by OBS

On Ubuntu, this can be done like this:

$ sudo apt install linux-headers-`uname -r` v4l2loopback-dkms

3. Load the v4l2loopback module

Installing the dkms package means that the module will be automatically loaded when restarting Linux, so this step only has to be done once.

$ sudo modprobe v4l2loopback

4. Configure the module
$ echo "options v4l2loopback devices=1 video_nr=10 card_label='OBS Virtual Camera' exclusive_caps=1" | sudo tee /etc/modprobe.d/v4l2loopback.conf
$ sudo modprobe v4l2loopback devices=1 video_nr=10 card_label='OBS Virtual Camera' exclusive_caps=1

5. Configure OBS

Open OBS and make sure that the virtual camera is stopped. Open the Settings menu, go to the Video tab, and set the Output resolution. Zoom only seems to accept certain resolutions. 640x480, 1280x720, 1920x1080 should be fine, but other resolutions might not be picked up by Zoom.

As a final minor difference, notice that OBS uses "Screen Capture" for capturing the screen rather than calling it "Display Capture" as in the screenshots for the other OS.