OXFORD University captain Kevin Brennan revealed the decision to leave lock Richard Graham on the bench was a very late call.
Graham started in 2005, but will be a replacement next Tuesday behind John Chance and Dylan Alexander.
Brennan said: "We didn't actually decide that until Sunday afternoon.
"Before he was injured at Bristol, Richard was playing really well, but John and Dylan came in and did excellently.
"We gave Richard his chance to get back in, but we felt we would kept the continuity with John and Dylan."
Five of Oxford's seven replacements are forwards.
Director of rugby Steve Hill said: "We felt we wanted our best 22 players available on the day.
"Both Marcel and James have pushed very hard for a starting place this year.
"We feel that those two and Richard Graham have the ability to come off the bench and make an impact.
"We also feel we have a degree of flexibility in the backs so only having two on the bench shouldn't be a problem."
Tomaszczyk's selection, although expected after a fine season, is a rarity as he is a first-year student.
Hill said: "It's a great achievement, especially as a prop. The last fresher that got anywhere near was Simon Danielli and he was only on the bench.
"Rob Andrew knew him up at Newcastle Falcons and said he was a real nuisance in training by getting in amongst the big guys and he has done that for us."
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