For room temperature or 4°C storage, oligos that are resuspended in TE buffer (10 mM Tris pH 8.0, 0.1 mM EDTA; such as IDTE), are more stable than dry oligos. Oligos that are stored in water are the least stable. However, at 4°C, oligos stored under all of these conditions are stable for at least 60 weeks. Also, 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.