The documentation isn't especially clear—CTRL+T is to jump back in the tag stack, whereas CTRL+O is to jump back to the previous cursor position. What's the difference between CTRL+T and CTRL+O? They both appear to exhibit the same behavior.
CTRL-T is working with tag stack
CTRL-O is working with jumplist
Tag stack and jumplist are different list in vim, but they might have same items when you jumping through tags (eg. using CTRL-])