Nucleic acids have a peak absorbance in the ultraviolet range at about 260 nm.
Extinction coefficients vary with the type of nucleic acid.
This is for a 1 cm path length. Therefore multiplication by 20 to give a 1 cm path length and then dividing by 20 for the extinction coefficient cancel each other. The A 320 is used to subtract absorbance due to particles in suspension in this small chamber. Count the numbers of each base in the oligo. Make your oligonucleotide solution more concentrated than needed so that when you add it to a working solution, the final concentration will be correct.