The unambiguous olive in the Old Testament is זית, záyit. The "oil tree", עץ שמן, ʿetz shémen, doesn't present us with such a certain translation. Literally, it is simply "oil tree", and there are three main candidates for it:

1. Olea europea, the olive tree.

2. Olea oleaster, the wild olive or oleaster. This tree is closely related to the olive tree, and is sometimes classified a a subspecies of the olive, O. europea.

3. Elaeagnus angustifolia, also called both the wild olive and oleaster (and also called the Russian olive). This tree is unrelated, but looks similar to the olive tree - thus the name.

A fourth candidate is the pine tree, as oil can be extracted from pine nuts.

All three of these produce fruit from which oil can be extracted. O. europea is more productive, and thus is the species used for commercial oil production since antiquity. All three grow in western Asia, and are candidates for being the "oil tree".

"Oil tree" is usually translated as olive. This is quite reasonable, since the olive is the most important of these oil trees. The word "oil", שמן, shémen, is often used to refer to olive oil (the identify being clear from the context, or on occasion occurring together with the word for olive as "olive oil"). However, Nehemiah 8:15 refers to "olive branches, oil tree branches, myrtle branches, and palm branches", and translating "oil tree" as "olive" would result in probably-wrong repetition. In this passage, "oil tree" is usually translated as "wild olive" (which of the two species, 2 or 3 above, is intended?). In the King James Version, it is translated as "pine", identifying the oil as pine-nut oil (turpentine is another pine oil, extracted by distillation from the wood, but this is a later technology).

The other appearances of "oil tree" are in Isaiah 41:19 and 1 Kings 6:23 and 1 Kings 6:31-33. In these cases, the conventional translation of "olive" doesn't produce any internal objections like the repetition in Nehemiah. In 1 Kings, where oil tree wood is used for doors, door frames, and statues of cherubim in Solomon's temple, it's possible that O. oleaster is meant, since this tree yields excellent wood, and harvesting it for wood doesn't sacrifice olive production. However, there isn't any evidence for (or against) this, and the usual translation of "olive" is quite reasonable.

(There are many other trees from which oil can be extracted, such as many nut trees. However, the olive, both types of "wild olive" or "oleaster", and the pine are the most likely due to being common in the area.)