Show remotes:
git remote -v
If you have not configured the upstream, it will not appear in this list.
To add the OSCAL repo as a remote named "upstream":
git remote add upstream https://github.com/usnistgov/OSCAL.git
git checkout master
git pull
git fetch upstream
git rebase upstream/master
First make sure that your local master branch is up-to-date with the upstream as described above.
Switch to the feature branch.
git checkout <feature-branch-name>