Use only one mathematical symbol and all numbers (0-9) to get a sum of 99

Write the next letter in this sequence.
O, T, T, F, F, S, S, E, __
**Hint: It is a Number Series Puzzle

How can you make a TV, a bed, a dog, and a car liquid?

Why do you find that most of the buildings are deprived of the 13th floor?

You are in a land where thirty men and two women were dressed in classic black and white dresses.
They begin to fight as soon as any of them move.

Where are you?

What is red and smells like blue paint?

A man who was outside in the rain without an umbrella or hat didn’t get a single hair on his head wet. Why?

I am not a bull but I have horns.
I am not an ass but I have a pack saddle.
Wherever I go, I leave silver behind me.

Who am I?

Which number comes next in the number series?

2, 9, 30, 93, ?

Accidentally, two trains are running in the opposite direction and enter a tunnel that is 200 miles long. A supersonic bird that has fled the lab and taken shelter in the tunnel starts flying from one train towards the other at a speed of 1000 mph. As soon as it reaches the second train, he starts flying back to avoid collision and meets the first train again at the other end. The bird keeps flying to and fro till the trains collide with each other.

What is the total distance that the supersonic bird has traveled till the trains collided?