Certified GKH Gardening Expert
This is to be expected with young growth. As the shrub ages, it will maintain a more rigid structure. For now, you can leave them, as they make an attractive drooping flower, or stake them up. Just put a bamboo stake next to it, and tie it, loosely, with soft string.
This article will help you with the care of the shrub: https://www.gardeningknowhow.com/ornamental/shrubs/oleander/oleander-growing-tips.htm