Secondary organic aerosol (SOA) and oxidized primary organic aerosol (OPOA) were produced in laboratory experiments from the oxidation of fourteen precursors representing atmospherically relevant biogenic and anthropogenic sources. The SOA and OPOA particles were generated via controlled exposure of precursors to OH radicals and/or O3 in a Potential Aerosol Mass (PAM) flow reactor over timescales equivalent to 1–20 days of atmospheric aging. Aerosol mass spectra of SOA and OPOA were measured with an Aerodyne aerosol mass spectrometer (AMS). The fraction of AMS signal at m/z = 43 and m/z = 44 (f43, f44), the hydrogen-to-carbon (H/C) ratio, and the oxygen-to-carbon (O/C) ratio of the SOA and OPOA were obtained, which are commonly used to characterize the level of oxidation of oxygenated organic aerosol (OOA). The results show that PAM-generated SOA and OPOA can reproduce and extend the observed f44–f43 composition beyond that of ambient OOA as measured by an AMS. Van Krevelen diagrams showing H/C ratio as a function of O/C ratio suggest an oxidation mechanism involving formation of carboxylic acids concurrent with fragmentation of carbon-carbon bonds. Cloud condensation nuclei (CCN) activity of PAM-generated SOA and OPOA was measured as a function of OH exposure and characterized as a function of O/C ratio. CCN activity of the SOA and OPOA, which was characterized in the form of the hygroscopicity parameter κorg, ranged from 8.4×10−4 to 0.28 over measured O/C ratios ranging from 0.05 to 1.42. This range of κorg and O/C ratio is significantly wider than has been previously obtained. To first order, the κorg-to-O/C relationship is well represented by a linear function of the form κorg = (0.18±0.04) ×O/C + 0.03, suggesting that a simple, semi-empirical parameterization of OOA hygroscopicity and oxidation level can be defined for use in chemistry and climate models.