The shelf life of an oligo is dependent on the temperature at which the oligo is stored, and how the oligo is resuspended. Temperature is the more important of the 2 variables. Generally, oligos should be stored at –20°C. At this temperature an oligo has a minimum shelf life of 2 years, whether it is stored dry / lyophilized, in TE buffer, or in (non-DEPC treated) water.
Please see the Technical Report, Oligonucleotide Stability Study, for data on oligonucleotide storage and a more thorough explanation.