Oral eroticism is experiencing sexual pleasure from the use of the mouth.
For instance, sexual pleasure is experienced when kissing someone. This is also related with performing oral coitus (hyperlink). In Freudian psychology, this refers to the oral stage of psychosexual development in which infants derive pleasure from using their mouths. Hence it is not exactly sexual in nature.
For instance, babies feel satisfied when they suck (especially in breastfeeding), bite, coo, and gurgle.