Olive tree (species name: Olea europaea) has a leaf arrangement that is a good example of the "Opposite & Decusatte" arrangement. The "opposite" part refers to the leaves arranged in pairs that grow out of the opposite sides of the stem. The "decussate" part means that the leaves are arranged on a stem in opposite pairs at right angles to those above or below. The images below give a pretty good perspective on what this looks like. In the case of the Olive tree the leaves themselves have a narrow lanceolate shape with a short petiole. These pictures also show the new inflorescence forming right above the petiole on the new branch growth.