The speedy Norwegian striker later had the ball in the net again, but was denied displaying his gymnastic prowess on a second occasion by the raising of the assistant's flag.
"It was the perfect ball into my feet behind their defence and I knew I was quick enough," said Alex on his goal. "I just used my pace to get one on one with the goalkeeper, then I tried to chip him and he made a good save, but the ball came back into my feet and straight into the goal.
"I was so happy that I did a somersault - it was such a good feeling," added Alex, who admitted that there were a few nerves around before the game, but now the focus is on a second meeting with Chester City in as many years in the next round.
"I remember the team losing to Chester last year and now we are going to meet them again. We were a little bit nervous before the game but once we got out there we were fine.
"I think we played really well after scoring the goal. We kept the ball, popped it around and made runs behind the defence. In my opinion my second wasn't offside, but it was called."
Alex Olsen reflected on the goal that sealed our Under-18 side's progress to the fourth round of the FA Youth Cup - and also prompted him to do a somersault.With an hour on the clock gone against Southampton at St Mary's, Alex capitalised on a precise throughball from Danny Rose and beat the keeper at the second attempt in what proved to be the decisive moment of the tie.