Low density polyethylene (LDPE) and polystyrene (PS) samples were functionalized with oregano´s essential oil (OEO) for possible antimicrobial packing applications. Different concentrations of OEO were used (0.5-1.5 % v/v). Polymers were UV light irradiated during 60 min to improve the OEO adhesion through the polar groups (COOH, OH or COO-) generation on their surface. The effect of OEO different concentration on their antimicrobial activity, mechanical properties and cytotoxicity of the functionalized polymers were analyzed. All functionalized samples showed antimicrobial activity against Gram positive and Gram negative bacteria and this activity increased with OEO concentration. The addition of OEO did not affect the LDPE mechanical properties. The flexural properties of PS were affected by the OEO highest concentration. A cytotoxic effect was found in samples with 1.0 % and 1.5% v/v OEO concentrations