For optimal stability, oligos that are to be stored long term should be stored frozen, at –20°C. If the oligos are going to be resuspended, storage in a TE buffer (10 mM Tris, 0.1 mM EDTA, pH 7.5–8.0; for example, IDTE) is better than using water (non-DEPC treated). The impact from the type of storage medium used is marginal at -20°C, but becomes more important at higher temperatures. It is ideal to store oligos in the dark. Exposure to UV light should be avoided, and ambient lab light should be minimized, particularly for some types of modified oligos.
Please see the Technical Report, Oligonucleotide Stability Study, for data on oligonucleotide storage and a more thorough explanation.