If you read Ludwig Wittgenstein's comments on language games, you will find that any next-in-series problem has an infinite number of soluitons. I am curious, though, to learn what you or others think is the "uniquely correct" answer to your challenge.
o
k
O
k comes next, and then l.
A is five letters before F Z is five letters after U G is five letters before L T is five letters after _ Answers is the letter O
The series alternates between letters in two patterns: the first letters (a, g) are separated by 6 letters, and the second letters (f, l) are separated by 6 letters as well. Continuing this pattern, after 'g' (the next letter would be 'm'), and after 'l' (the next letter would be 'r'). Thus, the next two letters in the series are "m" and "r", making the next sequence "m r".
According to SOWPODS (the combination of Scrabble dictionaries used around the world) there are 1 words with the pattern G-L-F--L-. That is, nine letter words with 1st letter G and 3rd letter L and 5th letter F and 8th letter L. In alphabetical order, they are: goldfield
According to SOWPODS (the combination of Scrabble dictionaries used around the world) there are 1 words with the pattern L-G--F-L. That is, eight letter words with 1st letter L and 3rd letter G and 6th letter F and 8th letter L. In alphabetical order, they are: lightful
According to SOWPODS (the combination of Scrabble dictionaries used around the world) there are 1 words with the pattern L-G-F-. That is, six letter words with 1st letter L and 3rd letter G and 5th letter F. In alphabetical order, they are: logoff
According to SOWPODS (the combination of Scrabble dictionaries used around the world) there are 2 words with the pattern -L-F----G. That is, nine letter words with 2nd letter L and 4th letter F and 9th letter G. In alphabetical order, they are: cliffhang cliffhung
According to SOWPODS (the combination of Scrabble dictionaries used around the world) there are 2 words with the pattern F----L-G. That is, eight letter words with 1st letter F and 6th letter L and 8th letter G. In alphabetical order, they are: fireplug footslog
According to SOWPODS (the combination of Scrabble dictionaries used around the world) there are 1 words with the pattern G--L-F--. That is, eight letter words with 1st letter G and 4th letter L and 6th letter F. In alphabetical order, they are: guileful