Fiala made it 3-2 with a shot to the corner at 2:21 of the third period, and O'Reilly scored a power-play goal at 6:16 to extend the lead to 4-2.
The Blues have scored at least one power-play goal in nine straight games (11-for-25, 44.0 percent).
Rask brought the Wild within 4-3 at 15:21 with a wrist shot from the right circle.
"What we saw last night was a great two periods and not a great third. Tonight we saw two terrible periods and a great third," Evason said. "It's encouraging that we know it's there. … We were awful at the start of the hockey game, and they took advantage of it.
"So, no, there's no concern, obviously. It's two teams playing hard, and if you cut it up it's probably pretty even."
NOTES:St. Louis forward Vladimir Tarasenko was a late scratch with a lower-body injury sustained Wednesday. "He is day to day, it's nothing worse than that," Berube said. … Blues defenseman Colton Parayko had two shots on goal and three blocked shots in 24:47 of ice time after missing two games with an upper-body injury. … Eriksson Ek has scored seven points (four goals, three assists) in an NHL career-high seven-game point streak. … With an assist, Wild forward Marcus Foligno has scored four points (two goals, two assists) in a three-game point streak. … Minnesota defenseman Jared Spurgeon had an assist, his 300th NHL point in his 701st NHL game. … Fiala has scored nine points (four goals, five assists) in the past six games.