**1. Introduction**

The *Origanum* genus covers a wide range of more than 60 plant species, most of which belong to the *Lamiaceae* and *Verbenaceae* families; among these species, *Origanum vulgare* L., *Lippia graveolens* Kunth, and *Lippia origanoides* Kunt are considered species of economic importance with a wide range of applications [1, 2]. Oregano essential oils (OEOs) are volatile compounds obtained from oregano leaves by steam distillation; they are composed of more than 50 phytochemically different molecules, such as terpenes, phenols, alcohols, organic acids, aldehydes, and ketones. Those metabolites are responsible for the medicinal properties of OEO, including antibiotic and anti-inflammatory properties that can help strengthen the immune system of birds [3]. The OEO may improve feed intake, the growth rate of broilers, and the overall body condition and quality of meat carcasses [4, 5]. Oregano is also

known to fight off bacteria that frequently make poultry sick, such as *Escherichia coli*, *Salmonella*, and *Clostridium* [2, 6]. The OEO is now used commercially by producers in place of antibiotic feed. Due to its antibiotic properties, oregano has been found to improve intestinal functions and alter the gut microbiota in broiler chickens and laying hens. Another application of OEO has been the improvement of intestinal antioxidant capacity, immunity, and intestinal microbiota in birds [7, 8]. Recently, the effects of oregano essential oil (OEO) on intestinal health and barrier function in poultry have been investigated. OEO has been shown to improve the barrier function of intestinal epithelium and consequently strengthen immune defense against pathogens in laying hens [8]. This mechanism appears to be based on regulating intestinal bacteria and inflammation. OEO has been shown to improve intestinal antioxidant capacity, immunity, and gut microbiota in yellow-feathered chickens and, therefore, improve growth performance, antioxidant status, and intestinal health in broilers. In pigs, the OEO has also been found to improve intestinal morphology and expression of tight junction (TJ) proteins associated with modulation of selected intestinal bacteria and immune status [9]. Furthermore, feeding OEO to pullets during rearing can help improve flock uniformity, reproductive fitness, and feed efficiency [8]. The OEO is known for its potent antimicrobial properties, which are primarily attributed to its major bioactive compounds, including carvacrol and thymol. These compounds possess significant biological activities and have been extensively studied for their antimicrobial, antioxidant, anti-inflammatory, and anticancer properties [10].

The previous evidence suggests that OEO has potential as a natural alternative to antibiotics for improving intestinal health, barrier function, and productive performance in the poultry industry. This chapter presents some experiences that we have had, as well as those of other authors, about the uses and applications of the different OEO chemo types for poultry production.
