Oguntibeju is one of two changes - the 23-year-old starts ahead of Max Williamson, while George Horne misses out through injury with Ben Ashfar getting the nod at scrum-half.
Explaining the call, Smith told Scotland Rugby News: "I think Jare is fresh and he's eager to contribute. To have some firepower from the bench will be important for us. We want to continue progressing.
"And at the same time, we're also looking a little bit longer term. We have three boys currently available, the other locks, with Alex Craig being injured and Scotty [Cummings] and Gregor Brown recovering. It's important that we share the workload properly amongst them."
Oguntibeju, a 2025 academy arrival and last season's breakthrough player of the year, has impressed rapidly. But Smith revealed a toe injury from his youth once affected his scrummaging.
He said: "When he came in here two years ago, he basically couldn't bend his toe. After an incident as a young boy, he kicked it into the sidewalk and he didn't rehabilitate it.
"That toe remained stiff, so it was tough for him to scrum off that. He scrummed off the side of his foot. With the diligent work from himself and our medical team - even though he's had a big knee injury - to get that mobility going and to move better took a lot of work.
"There was pain sometimes, especially at scrum time, but now he's pushing off that toe and he's progressed so well, he's running freely. And I already tasked him with calling the lineouts a little bit against the Scarlets to give him confidence.
"He's progressed and he's slowly but surely becoming an athlete. He's applying himself with and without the ball well.
"He also is somebody that - like Ben Afshar - has got all the potential to step up and to look for more responsibility, and in games like this, they will learn and experience and it will help their development."
Horne's absence means Ben Afshar starts, with Jack Oliver on the bench. Smith remains confident in both young scrum-halves.
He said: "George has obviously not recovered. He's in a boot and as much as we tried to get him ready for this week, I think we've made a clever decision to trust the boys that we've got.
"I think both Ben and Jack have proven over the last six or seven months that they are ready to take that baton, and I'm very excited about that.
"And to play in a quarter-final for these boys, it's obviously the next ingredient, as it is for Dan Lancaster. To get this experience, to play in play-offs, where it's obviously a different atmosphere and pressure, it's an experience that they will take forward in their career.
"So we're really excited. Honestly, I've got all the confidence in the world. Jack has been pushing hard for a starting berth as well, so both of them have made this big step up.
"I'm excited to see the progression. It will lead to some new heroes, hopefully, that can take the baton."