Obasanjo actually calls the President "Brother Buhari" in his warning letter

“I only appeal to brother Buhari to consider a deserved rest at this point in time and at this age. I continue to wish him robust health to enjoy his retirement from active public service. President Buhari does not necessarily need to heed my advice. But whether or not he heeds it, Nigeria needs to move on and move forward,” he said.

Wow! I think that's not okay to address the president!!