For long term oligo storage, temperature is the most important factor to consider. For long term storage, whether oligos are dried down, or resuspended in non-DEPC treated water or TE buffer (10 mM Tris pH 8.0, 0.1 mM EDTA; such as IDTE), it’s optimal to store them frozen, at –20°C.
For room temperature or 4°C storage, oligos that are resuspended in TE buffer are more stable than dry oligos. Oligos that are stored in water (non-DEPC treated) are the least stable. However, at 4°C oligos stored under all of these conditions are stable for at least 60 weeks.
Please see the Technical Report, Oligonucleotide Stability Study, for data on oligonucleotide storage and a more thorough explanation.