The visitors were dismissed for a paltry total of 77 after being inserted on a slow surface.
The Volts did not exactly blast their way to victory, though. Leg spinner Ish Sodhi saw to that. He kept the top order pinned down and bowled unchanged from the media end to take one for 16.
But once he was out of the attack, Otago made more rapid progress and finished with a swagger.
Brad Wilson hit consecutive sixes off Anton Devcich to seal the win with 8.3 overs to spare.
The home side owed its comprehensive victory to some tight bowling.
Captain Nathan McCullum (two for 13) was instrumental. He picked up two wickets in the ninth over with his off spin to reduce Northern to 41 for five.
Fast bowler Warren Barnes (one for 18) was also impressive. He has an accurate yorker, a useful bouncer but generally just bowled with good heat.
Left-armer Bradley Scott proved difficult to get away again but Finnie was the star.
Until yesterday, the 18-year-old had bowled just one over, which was smashed for 12 runs, and had also batted once, nicking out for one.
But the promising teenager made a telling contribution, snaring four for 11 from four quite wonderful overs.
''I'm real happy to finally get a crack,'' he said.
''It was a nice time to come on and have a crack at the tail end. Some of their better players had gone out.
''I've been bowling pretty well in the nets and so I've just been making sure, when I do get the chance, that I'm ready.''
The off spinner came into the attack in the 12th over and at a nice time to bowl.
The visitors were five down and Finnie struck third ball, removing the dangerous Devcich.
He was the pick of the Northern batsmen and had got through to 33. But with five wickets having tumbled at the other end, and the overs slipping by quickly, he chanced his hand and miscued, holing out at backward point.
''I was pretty happy to get him out.''
With Devcich gone, Scott Kuggeleijn shaped as the mostly likely remaining player to get Northern through to anything remotely defendable.
He pummelled a drive that threatened to pass between Finnie's legs, only for the bowler to intercept the ball about shin height and claim a cracking caught and bowled.
He took consecutive wickets in the 18th over. Both batsmen swung and missed. The bowler did not.
Sodhi saw off the hat-trick ball with a well-placed pad and survived the lbw appeal.
Otago has won three matches in a row and four from five. The Volts sit at the top of the competition standings with 16 points. Their next match is against Auckland at Eden Park Outer Oval on Sunday afternoon.
''We're obviously feeling pretty good about the way we are playing at the moment. It is all sort of going our way, but we know we are going to get into situations which might not go our way and we just have to try and get through them.''
