A Letter

I have listed two names alphabetically,

But I dare not call a letter an alphabet

For the letter, a part, is part of the alphabet, a whole.

Also, the grammar gods could get angry and say,

"This an error is also terror!"
Change only one letter, my friend,
I am not asking for anything more.
Notice how your mind reacts,
On changing a B to an S
In the names of two famous men given below:
Obama and Osama