To initialize your local repository using the OMNIROM trees to build TWRP, use a command like this:
repo init -u git://github.com/minimal-manifest-twrp/platform_manifest_twrp_omni.git -b twrp-8.1
export ALLOW_MISSING_DEPENDENCIES=true; . build/envsetup.sh; lunch omni_-eng; mka recoveryimage