Write the stick

one stick, a few minutes. then hold Option.

You need a USB stick of 2 GB or more (8 GB for OmaBoot? Omarchy). Everything on it is erased.

The image is about 2 GB. The first time you save a report to the stick, OmaBoot? Live adds a small partition for reports in the space left over.

Get the .iso from Download and verify it first.

macOS

Easiest: balenaEtcher. Choose Flash from file, then the stick, then Flash.

Or in Terminal with dd. First find the stick's disk number:

diskutil list external

Check the size and name twice. dd overwrites whatever disk you give it, without asking. Then, with your number in place of N:

diskutil unmountDisk /dev/diskN
sudo dd if=omaboot-live-<version>.iso of=/dev/rdiskN bs=4m
diskutil eject /dev/diskN

If macOS says it can't read the disk, choose Ignore.

Linux

With GNOME Disks: pick the stick, open the menu, choose Restore Disk Image…, pick the .iso and start.

Or with dd. Find the stick (TRAN says usb), and check it twice:

lsblk -o NAME,SIZE,MODEL,TRAN

sudo dd if=omaboot-live-<version>.iso of=/dev/sdX bs=4M status=progress oflag=sync

Use the whole disk (/dev/sdX), not a partition (/dev/sdX1).

Windows

Use balenaEtcher as on macOS, or Rufus: pick the stick and the .iso, then Start. When Rufus asks how to write the image, choose Write in DD Image mode.

Boot it

Plug the stick straight into the Mac, not into a hub. On a laptop, plug in the charger too.
Turn the Mac on and hold Option (Alt on a non-Apple keyboard) until the startup disks appear.
Pick OmaBoot? Live and press Return. A short menu shows for 5 seconds, and its first entry starts the tests.

This choice is for this start only. Next time, the Mac starts from its own disk as usual.

If the stick doesn't show up, try another USB port or another stick. Some older Macs don't list every stick.
If the Mac has a firmware password, it asks for it first.
Macs with the T2 chip (2018–2020) only start from a USB stick once you allow it. See T2 Macs.

When you're done, shut down and pull the stick.