The big Scottish winger had his hat-trick done and dusted by the 53rd minute. By that time, the visitors had the bonus point in the bag.
Benetton Treviso made a valiant effort to close in on a losing bonus point, with their scrum half Tobie Botes sniping through for two tries in the final quarter.
But the Ospreys were too far ahead for those scores to matter and the defending champions are now back up to third in the table.
Treviso were back in action for their second match in six days. Their coach Franco Smith made two changes to the team that saw off Munster at this venue last weekend.
The switches were in the front row where Michele Rizzo and Lorenzo Cittadini made way for Ignacio Fernandez-Rouyet and Pedro di Santo.
The Ospreys had Walker back from international duty and fellow winger Tommy Bowe also made his return from a knee problem which surfaced last month.
Although they were missing 10 Wales internationals and a further seven injured players, the Welsh region made a confidence start.
Dan Biggar struck a penalty off the post, but almost immediately the visitors were back on the attack and Walker finished off a move involving Biggar and Richard Fussell in the corner.
The try was unconverted, but Biggar was back on form when adding the extras to Ian Gough's 17th-minute try, his first in Ospreys colours.
Biggar's inside pass set up the big lock for a long-awaited try. Botes had missed a penalty before that, however it was mostly one-way traffic in the opening half hour.
A third try followed for the Welshmen with 29 minutes on the clock. Treviso failed to clear their lines and centre Ashley Beck was well-placed to pounce and score. Biggar watched his conversion attempt come back off an upright.
Botes had better luck at the other end, clipping over two penalties to close the half out at 17-6. Treviso really needed to score first after the break, but against it was the Ospreys who took control.
Eight minutes in, Bowe managed to win a good turnover for his side and the ball was ferried wide for Walker to punish Treviso with the bonus point try. Biggar also landed the conversion from a difficult position.
There was no let-up from the visitors. Scrum half Rhys Webb broke Treviso's defensive line to set up Walker's third of the evening. Biggar converted and tagged on a penalty to make it a 34-6 deficit for the Italians.
Both benches were used quite regularly as the game entered the final quarter and Treviso rediscovered some of the form which saw them beat table toppers Munster last time out.
Botes converted his own try in the 66th minute and finished off a fantastic length-of-the-pitch effort for his and Treviso's second touchdown.
Botes' second came shortly after Treviso number Marco Filippucci had been sin-binned. Smith's side battled right until the final whistle, but the Ospreys were able to close out a satisfying victory - their first on the road in the league since November.
Their head coach Sean Holley said: "It was an excellent team performance, we were clinical when we needed to be.
"I thought we attacked with real purpose throughout, the feel and mood of the team is very good at present. The players were diligent all week, we were well prepared for this game, showing Treviso the respect they deserve.
"This is not going to be an easy place for teams to come so to get the bonus point too was extremely satisfying. Treviso defended well, especially in their own 22, and thwarted some good attacks.
"Their set piece was strong but I felt that the presence of Ian Evans in the lineout was a factor. There were some fine individual performances, with Nikki's three tries a highlight."
|Benetton Treviso Score Card|
|Ospreys Score Card|