We recommend dissolving the stock oligo in concentrated form in TE (10 mM Tris pH 8.0, 1 mM EDTA). Alternatively, sterile dH2O can be used. DNA kept frozen in a nuclease-free environment should be stable for years. We find it convenient to initially make a freezer stock (which should be thawed relatively infrequently) at 100 µM concentration. Adding a volume of TE (µl) equal to ten times the number of nanomoles of DNA present in the tube (as noted on the spec sheet provided with the oligo) will produce a stock solution at this concentration. For example, dissolve 50 nmoles of oligo in 500 µl TE to make a stock 100 µM solution. Dilute from this stock 1:10 (in water) to make a working solution at 10 µM for use in setting up PCR reactions. Most PCR reactions use 0.1 - 0.5 µM primer. Addition of 1 µl of the 10 µM primer to a 20 µl PCR reaction will result in a final primer concentration of 0.5 µM, or 10 picomoles of oligo in 20 µl volume.
You can also refer to our Resuspension Calculator Software online. This program will help you to quickly determine how much buffer/water to add to the oligo to meet your desired concentration. Simply enter the number of nmoles in your vial, change the drop down button from "OD260" to "nmoles", type in your desired concentration in the appropriate space, and hit calculate. Here is the link to this program: http://www.idtdna.com/analyzer/Applications/ResuspensionCalc/