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Pig production depends on a health and performance balance. An approach to improve intestinal health is the oregano essential oil (OEO) supplementation within a conventional diet. Intestinal integrity regulating effects, for example gene expression, of some feed ingredients are important key factors for that balance. We hypothesized that OEO affects the expression of genes associated with pigs' intestinal integrity. In four trials, a total of 86 pigs have been used. From weaning, the 'treated' group (n = 42) was additionally fed an oregano flavour additive [1500 mg/kg (7.5% pure OEO)] within the basal diet. The 'control' group (n = 44) was kept under identical environmental conditions, except the OEO. At age of 6 months, pigs were slaughtered with an average weight of 111.1 ± 10.9 kg. In addition to automatically generated 'Fat‐o‐Meter' (AutoFOM) data, carcass quality factors have been measured manually. Valuable cuts of meat, such as ham and belly, were significantly reduced in the OEO group. Effects of OEO on pigs' haematologic parameters were very limited. For transcriptome analysis, the most interesting microarray expression results have been listed in a table (topTable). Selected genes were technically validated by qPCR. As a result, few significant differences in animal development and meat quality have been found between the OEO treated and the control group. Depending on OEO supplementation, we found 93 differently regulated genes in the jejunal tissue (70 up, 23 down) and 60 in the ileal tissue (48 up, 12 down). Just three genes (GRIN3B [glutamate ionotropic receptor NMDA type subunit 3B], TJP1/ZO‐1 [tight junction protein ZO‐1] and one uncharacterized gene) were affected by OEO both in jejunum and ileum. qPCR validation revealed AKT serine/threonine kinase 3 (AKT3), Interferon (IFN) ‐ε, ‐ω, tight junction protein (TJP1)/ZO‐1 (ZO‐1) to be upregulated in the jejunum and C‐C motif chemokine ligand 21 (CCL21) was upregulated in the ileum of pigs that were supplemented with OEO. OEO supplementation had limited effects on pigs' performance traits. However, we were able to demonstrate that OEO alters the expression of genes associated with adaptive immune response in pigs' small intestine. These findings help to explain OEOs' beneficial impact on pigs' intestinal integrity. [ABSTRACT FROM AUTHOR]