Once you have completed the creation of your Ad Strategy, you will still be able to make the following changes by navigating to the gear icon in the top right on your individual OptiGroups page:
Alter the OptiGroup name
Change the goal the campaigns in the OptiGroup are optimized against
Add a campaign to an existing OptiGroup, as long as they don't exceed 20 total
Remove a campaign from an existing OptiGroup, as long as there are at least 2 live campaigns remaining
Don't forget to hit the Finish button to save your changes.