Overstaining does not depend on the gel %. You have to stop reaction (by means of acetic acid) right after first bands appear.

With denaturing gels 20percent gel is good for oligos 8 - 25 bases and 10 percent is good for 25-35 mers so it depends on the length of your oligo and also if you are running native non denaturing gels possibly also on the sequence of the oligo in part but andrew is right that the detection will continue unless you stop it 

you could also try staining with methylene blue. method lifted fro qiagen publication below

Detection of oligonucleotides

Oligonucleotides in polyacrylamide gels can easily be detected by staining with 0.02% methylene

blue staining solution.

1. Remove the gel from the glass plates and put it into a plastic box (which should be

slightly larger than the gel).

2. Add 0.02% methylene blue staining solution to the box.

3. Agitate the gel gently in the solution for 20–30 min.

Oligos should become visible after 10–15 min. If staining is weak after this time, add fresh

methylene blue staining solution.

4. Remove the staining solution (methylene blue staining solution may be stored for

later reuse).

5. Destain the gel by adding distilled water to the box. Leave gel submerged in water

for 2–3 min and replace with fresh water when it becomes deeply colored with the

dye. Repeat 2–3 times until the blue background of the gel is sufficiently reduced.

Note: Silver staining, using commercially available reagents, can also be used for detection

of oligonucleotides.

Note: We do not recommend the use of ethidium bromide for staining of oligonucleotides,

since staining intensity varies depending on the sequence of the oligonucleotide.

6. Photograph the gel for documentation.

but I think this is less sensitive than silver staining