To my mind, it is up to how you evaluate. If only grades are given to students, an oral presentation can be considered as summative assessment. If you give feedback on students' performance showing what they did good, moderate and/or bad in detail, it could be a formative assessment tool.
It may be both, as Mehmet clearly explains. I would just add that if the presentation is part of the formative assessment and delivered in groups, try to assign roles to the members of the group. In this way, different marks can be given to each member. It's also fairer.
If you elect to use oral assessment summative ly, I would suggest having a panel of three or four assessors if at all possible. Unless you record every presentation, there is no way of reviewing an oral assessment
I agree that it depends on the intended purpose. When the purpose is improvement, that is formative. If the purpose is judgement or determination of value, that is, summative. I love the saying from the work of Robert Stake:
“When the cook tastes the soup, that's formative; when the guests taste the soup, that's summative.” (Scriven, 1991, p.169)