Oil palm tree counting is a quite simple application. You can simply use following methods:
1. Ravine extraction (Honda Kyioshi): Based on the fact that the top canopy is the brightest point. The ravine (or the valley between 2 trees) looks darker. Doing it, you wil be able to locate the central point of each tree. Then counting is some simple works to do aftewards.
2. Local threshold texture matching: Requires some programming tasks. Get the individual trees as samplings (with different sizes of canopy) and then match them over the whole image. The matching algorithm can be image correlation or histograming matching. I already did it with VB programming and it works perfectly. By doing this, you can also be able to map individual trees.
All of this is based on RGB image. For better results, mask the plantation areas before running the procedure.
Of course there are several other options to do. If you want to operate the procedure in an automatic way, the programming skill is needed.