Marlborough (Woodstock) battled to a 7-5 win over St Birinus (Didcot) in the Oxfordshire Under 14 Cup semi-final.
A try by Tom van der Ploeg, converted by Jake Willis, gave Marlborough the edge.
They now await the winners of Lord Williams's (Thame) and Burford in the final.
Magdalen College School (Oxford) overpowered Shiplake College (Henley) 29-13 in the Oxfordshire Under 19 Cup quarter-finals.
Robbie Blencowe, George Badger and William Hammond crossed the Shiplake line, and Magdalen were also awarded a penalty try. Hammond landed two conversions.
Now Magdalen face Cokethorpe (Witney) in the semi-finals on Wednesday when Henley College also entertain Lord Williams's.
In the Oxfordshire Under 15 Cup semi-finals, Gillott's entertain Henley rivals Shiplake, while Cokethorpe are at home to Lord Williams's.
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