When determining whether to use sunscreen or insect repellent first, always apply sunscreen first, followed by your insect repellent. It’s recommended that people reapply their sunscreen more frequently. No need to reapply your insect repellent more than once per day.
The same logic applies to natural repellents, such as oil of lemon eucalyptus (OLE). Apply a layer of sunscreen first, wait for the sunscreen to dry, then apply the oil of lemon eucalyptus to your skin.