The 20-year-old, impressive in a rewarding season-long loan at near-neighbours Bradford City last term, has signed fresh terms which commit his future to the Terriers until the end of June 2022, with the club holding the option of a further year.
O’Brien is now expected to push for a first-team place, with Siewert admitting the youngster has caught his eye in pre-season.
Siewert said: “It is great news for everyone that Lewis has signed his new contract.
“I think that everyone has been able to see the benefits that his experiences at Bradford City last season have given him.
“He has come back to us with his confidence high and he can fit into our squad with his level of performance, as we have seen in pre-season.
“He is still only 20, so he definitely has space to improve further. That is an exciting thought, as we have already seen he can make an impact for us.”
Leeds United forward Patrick Bamford has revealed his regret at the controversial incident with led to him receiving a two-game ban for deceiving a referee against Aston Villa late last season.
Bamford went down under minimal contact during a confrontation with Anwar El Ghazi in the 1-1 draw at Elland Road in late April, with the Villa winger being sent off as a result.
The decision was overturned following an appeal by Villa and Bamford missed Leeds’s final regular game of the season and the first leg of the Championship play-off semi-final with Derby.
Admitting he must avoid a reputation for “milking” decisions, the striker said: “Once I have put that in people’s minds, it is going to be hard to change that.
“But it is something I have got to do over the next few years. Looking back, I regret it. I maintain he touched me, but I milked it and made a stupid deal of it.
“I would not do it again and it is something I have to learn from.”
On regaining the trust of United supporters, many of whom were critical of his role in the incident against Villa, he added: “I took a bit of stick last year from the fans.
“There will always be people who like you or are not such fans (of you), and for me it is an opportunity to prove them wrong and get them on my side.”