How are oligos quantified?

Oligonucleotides are most commonly quantified by measuring their UV absorbance at 260 nm. A single-stranded oligonucleotide dissolved in a neutral aqueous solution at a concentration of 33 µg/ml will have an absorbance at 260 nm of approximately 1.0 A.U. The optical density (OD) unit is defined as the absorbance (at 260 nm in a 1 cm cuvette) multiplied by the sample volume (in milliliters), so that 1 OD unit is roughly 33 µg of ssDNA.