Glamorgan closed Day 3 on 137/1, leading Worcestershire by 216 runs thanks to Labuschagne (90*) and Selman (42*).
Labuschagne also became the first player in the country to reach 1,000 Specsavers County Championship runs in the process, as he reached his ninth score of 50+ on the season.
Michael Hogan had earlier taken his 23rd five-wicket haul in first-class cricket to help dismiss Worcestershire to 370, with his 5-62 from 29 overs leading the way as he took his tally of County Championship wickets to 25 in 2019.
"We’re in a good position," said Hogan. "It would have been nice to finish off the Worcestershire tail a bit quicker but D’Oliveira’s played nicely there, a gem of an innings.
"I think we’re still ahead of the game but it’s going to be harder work to win the game than we would have liked.
"We possibly went away from our plan a bit, maybe bowled a bit too short on a decent wicket, and that created a bit of frustration for us.
"For the most part, I think the boys stuck to the task quite well.
"I'm happy with the way the ball is coming out. For a few of our boys the ball is coming out nicely and as a group the attack is going really well, so luckily for me I'm taking the wickets at the moment but it is a collective and we are all building pressure."