For me, seeing things side by side can really help clarify things. Here are the leaves of the different ‘Oha wai known with certainty from O’ahu. From left to right: Clermontia kakeana, Clermontia oblongifolia and Clermontia perscifolia. The leaves are all from plants roughly about a year and a half in age.
Immediately appreciably is the greater size of C. kakeana. But as we look closer, other differences become more apparent. C. persicifolia is a much deeper shade of green. On the live plant, the leaves are much glossier too.
Looking at C. persicifolia, we can see that the venations is reticulated and also impressed giving the upper leaf surface much more texture.
For the Clermontia oblongifolia population on Pu’u ‘Ohia, the petioles tend to be much longer than the other 2 species. In this population, perhaps because of this, C. oblongifolia leaves tend to be much droopier.
The abaxial surfaces are interesting as well. Again on C. persicifolia, the venation is much more conspicuous. What we can’t see from this picture (Still working on purchasing a macro lens) is that underside of C. kakeana leaves are quite hairy. The other 2 are much more glabrous.
And of course, most traits in nature are on a gradient. I’m sure you’ll run into an ‘oha wai on the trails that doesn’t quite fit these basic keys. Still I hope this helps with id’ing these awesome members of the forests of O’ahu.