The first step is to upgrade your BootROM, read the first post of the thread below:
Please fully read this first post, you will probably find that you have one or more problems described into the various notes below. Mojave will only install if you have upgraded your BootROM to the current release and your Mac Pro have a Metal capable GPU*. If you are trying to install Mojave...
forums.macrumors.com
The second one is to be sure that you have a Westmere Xeon on your Mac Pro: E56xx, L56xx, X56xx, W36xx. While OpenCore works with older Nehalem Xeons, you won't have access to one of the best resources, VMM spoofing and native software updates.
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If you have any doubts about OpenCore install, please ask on the OC thread, there are lots of people that can help you there.