Is this gall on my oak leaf?
You are looking at oak gall from cynipid wasps. Many different galls can develop on the same kinds of plants.
Galls are growing plant parts and require nutrients just like other plant parts. It is possible that galls steal vital plant food and adversely affect plant growth. This is most likely a problem when galls are numerous on very young plants. Injury may also occur if galls are numerous on branches, or if abundant for several consecutive years. In most cases, however, galls are not abundant enough to harm the plant.
Although cosmetically unappealing, most galls do not adversely affect plant health. Therefore, management is generally not suggested.