The 26-year-old appeared in 83 games with the Penguins since the trade, recording 25 points (eight goals, 17 assists), including four goals and seven assists in 36 games this season.
"We've been watching him closely, and he's a more mature, well-rounded player," Nill said. "He's more calm in situations, his defensive reads, not as many highs and as many lows. There's real consistency."
Oleksiak, who is signed through 2020-21 with a cap hit of $2.1375 million, had his ups and downs during his first stint with Dallas and was never able to crack the lineup on a regular basis. The highest number of games he played in a season in Dallas was 41. Before being traded in 2017-18, he was a healthy scratch in 21 of 34 games with Dallas.
But Nill said Oleksiak has grown as a player following the 2017 trade, a fresh start and change of scenery that Oleksiak used to his advantage.
"He went to another team -- sometimes that has to happen -- and he's become a better player," Nill said. "Pittsburgh is a very good team, and that rubs off on you. But he's become a better player, and you have to give him credit."
Oleksiak played in every game with Pittsburgh the rest of the 2017-18 season and in all 12 playoff games following the trade. This season, with the Penguins deep on defense, he's played in 36 of 48 games, missing three with injury and sitting out the other nine as a scratch. Pittsburgh had been carrying eight defensemen and was about to have nine with Justin Schultz about to return from a leg injury.
That glut of defensemen had Pittsburgh GM Jim Rutherford looking to move one, and with the Stars needing help on the blue line, the two sides came together. Nill said the uncertainty of what might happen at the trade deadline played a factor in the timing as well. There are too many teams still in the playoff hunt and teams are still trying to lock up players who could be late season rentals.
"I don't know where the trade market is going to go," Nill said. "All of a sudden one or two guys sign with their teams or one or two guys get traded, and I don't know what else is going to be out there. This was a chance for us to get ahead of the curve a little. Pittsburgh was in a situation where they had nine defensemen. Something was going to happen for them and it just kind of worked in our favor."