Linear, branched and crosslinked polyethylenes (PE) were exposed to the low-pressure oxygen plasma for 2-120 s. In the following the samples were washed with solvents to remove low-molecular weight oxidized material and to excavate the subjacent polymer structure for microscopic characterization. X-ray photoelectron spectroscopy (XPS) measurements provided information about changes in elemental composition and chemical structure of PE after plasma exposure and washing. The calculation of the concentration of tertiary C atoms using XPS data was a measure of branches and crosslinking in the polymer before and after exposure to oxygen plasma. Linear PE was most sensitive towards oxygen plasma and showed the highest concentration in tertiary C atoms after plasma exposure. On the other hand branched PE types, which possess originally more tertiary carbon atoms, have lost two-third of them after 2 s oxygen plasma exposure. Branched PE show also topological changes at their surface as detected by atomic force microscopy. Differential scanning calorimetry measurements confirmed strong changes in crystallinity and molecular orientation of linear PE already after 120 s exposure to the oxygen plasma interpreted as amorphization. These effects should be interpreted as result of crosslinking caused by the recombination of dangling bond sites.