The Norwegian centre half is on loan at the German second division club for the season and helped them claim their second 2-0 win of the season over arch-rivals Hamburg SV on Saturday.
He said, “That’s the best game of my career – just ahead of the game against Hamburg we won back in September, it was really quite special.
“To win in front of 60,000 people was incredible and the atmosphere was just unbelievable - it was my biggest game so far.
“We’re history-makers now – no FC St Pauli team had beaten Hamburg twice in a season until now. It’s massive for everyone in the city who supports the club. This game is the most important of the season for them.
“In the days leading up to the game you felt how big it was, you knew it was more than just a normal game, it’s like a final and there was a real countdown before it.
“You felt it in the warm-up when you looked around and saw the crowd that it was a really big occasion.
“After the game the team went to eat together in a restaurant, all the fans were there and sang songs with us. That was amazing. It’s important to do that when you win this kind of match.
“The fans are incredible, they’re with us every game and they deserved the win. Hamburg is brown and white this year and that’s a great feeling!”
Ostigard struggled with an injury at the start of the season, but after making his debut on the last day of August has gone on to make a further 18 league appearances.
The 20-year-old says it’s been an eye-opening experience, and one that he believes will stand him in good stead for the rest of his career.
“I’m feeling really good, playing a lot of matches and I feel like I’m developing all of the time. This has been the perfect step for me to continue to improve,” he said.
“There’s been a whole host of experiences in front of big crowds and high-pressure environments – just like the Premier League in many ways. It tests you every week because there’s a lot of stake. You have to fight in every game to help the team climb the table.
“But you learn a lot from that – sometimes you get more from being in a side that doesn’t win every week. It’s good that I have learned a lot so early on in my career - when things aren’t going as you want, and you have to step up and fight.”
Ostigard isn’t the only Albion face to be plying his trade with a club that is well-renowned for its distinctive social culture - Swedish international Viktor Gyokeres also made the switch during the summer.
“Viktor has been happy with the year so far too,” said Ostigard. “He hasn’t played as much as he would have liked perhaps in recent weeks, but it’s been a good step for him as well.
“He scored a lot of goals in a short period of time before the winter break and he’s been an important player for us.
“We are close friends and I really want him to keep doing well here. We are enjoying it and are close off the field, all we can both do is keep going and doing our best.”