Definitely not Oleaceae. Swollen petiole base suggests it is a compound leaf. Is this the max height? Do you have images of open flowers? Also do you have images of stipules or fruit?
yes i agree it is not oleaceae. yes 40 cm is max. height observed among population. The flowers open this much only, i have some other views and closeup which i will add here no fruits were found. stipules image is not present with me.
I dont have much idea but it looks like Euchresta formosana sp. more but the only difference to me is their leaves and they are shrubs while you are saying its herbaceous plant.