Storage of RNA lyophilized at -80°C is the ideal, however storage at -20°C is acceptable. If you do not have a way to dry down aliquoted RNA, the next best is to store the oligo as an ethanol precipitate or in RNase-Free Water or TE at -80°C or -20°C in RNase-Free tubes.
DEPC Water is not recommended. While it makes solutions "nuclease free" most labs are not able to remove all of the DEPC (released as CO2
after autoclaving). Some will stick to the glass bottle and will, in long term storage, degrade the RNA. There are commercially available storage buffers, such as the sodium citrate storage buffer available from Ambion
We recommend that all reagents you use be tested with Rnase Alert to ensure that you are using RNase-Free reagents.