Each laser machine will be different with settings. As Ferg said above, you need to do your own testing (speed, power, frequency and focus distance).
I have a 50 watt CO2 (not omtech). For cutting, air assist is a must. I mostly cut 6-7oz veg-tan. Power is set to 100% and speed is set to what will cut through it.
For engraving, I have settings with and without masking tape. Power and speed settings are roughly in the middle. If you want a darker engrave, you need to go out of focus a little.
Make sure you save all your settings.