You cannot enable or disable change tracking globally. You have to do it in topic by topic.
Procedure
Check out the topic open in Oxygen XML Author.
To enable track changes, do one of the following:
Click Edit > Review > Track Changes.
From the toolbar, click .
Right-click and select Review > Track Changes.
As you make changes to the topic text, the changes are highlighted.
To turn off tracking changes, from the toolbar, click .
Turning off change tracking does not remove the change markings for any changes you made to the topic while change tracking was enabled. You must accept or reject the changes to clear the change marking.