If a given DNA oligo is an aptamer, will the corresponding RNA oligo with the same sequence be an aptamer aswell?
There is at least one known case where both the DNA and RNA versions of the same aptamer sequence bind the same target (Lauhon and Szostak, 1995).

But you can't generalize this, there are two differences in structure between RNA and DNA, the Uracil/Thymine change and the missing 2'-OH in DNA. Those differences can affect the binding and overall structure of the aptamer. 

It is possible that the RNA version of a DNA aptamer binds the same ligand, but this won't be always the case.