The duo struck from long range inside half an hour to put the League Two leaders on course for another win.
And it left them needing just one more point to be sure of automatic promotion, ahead of a midweek trip to Gillingham.
Sadlier opened the scoring with a superb free-kick before James added to his outstanding collection with a top-corner finish.
“The two goals are just unbelievable talented moments,” Wellens told the club website.
“Sads is talented and to whip it like that was a fantastic goal and TJ we’ve obviously come to see a few of them.
“So frustrated we’ve won the game through two worldies, but we should be winning that game four, five.
“The keeper has pulled off some good saves, we’ve missed one-on-ones, so it’s a little bit frustrating, but we came here to do a job, a clean sheet and the game changed a little bit second half because of the 2-0 scoreline and the 2-0 scoreline against Harrogate but last week, it happens.
“You see Manchester United in the Europa League, teams change and games can change if you’re not doing the basics right.”
O’s players celebrated after the final whistle, with there being some uncertainty about whether or not they had already clinched a place in League One.
They had a 13-point cushion over fourth place and Wellens added: “The way the crowd responded we thought we was up. Then we found out we wasn’t up.
“You want to celebrate but then you want to get off the pitch and be professional and make sure we’re ready for Gillingham.
“In terms of the game, tactically very good, really good performance, little bit frustrated because the way that we played should’ve brought more goals.
“We did the basics right, we changed a little bit and got sloppy at times, in possession second half, but the basics, long throws, set plays, corners against this team is important and we were very good at that.”
It also saw goalkeeper Lawrence Vigouroux set a new record with his 23rd clean sheet and he was quick to praise those in front of him.
“I’m delighted to have broken that record, it’s been standing for a long time,” he said.
“I can’t take all the credit, a lot of it has to go to my teammates, they defend really well.
“Luckily we kept another clean sheet here today and another three points which is the most important thing.
“All the boys were seeing Sky Sports saying we were up. We’ve got to go to Gillingham on Tuesday, it’s another big game, we’ll another great following. I thought the following today was brilliant so we’ve got to thank them.
“We go to Gillingham and hopefully we can another three points there just to cement it.”
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