Rashi: It is the oil with which Aharon was anointed. "Ka'Shemen ha'Tov; k'Tal Chermon" - this is like this, like "cha'Am ka'Kohen" (Yeshayah 24:2).

Radak: There is a Patach under the Kuf; it is the known oil, i.e. Shemen ha'Mishchah. Just like the anointment of Aharon was good and pleasant, so will be the dwelling of brothers (refer to 133:1:2:4).It mentions anointment of the Kohen, but not of David ha'Melech, for two reasons. (a) Aharon was anointed first, when honor dwelled in Yisrael, when the Mishkan was erected. So in the future, when the Mikdash will be built, and Melech ha'Mashi'ach and the Kohen Gadol will rise, the honor will dwell there. (b) Aharon was anointed in front of all of Yisrael; David was anointed covertly in front of Yishai, his sons and the elders of Beis Lechem. In the future it will be exposed in front of all of Yisrael and the nations.

Malbim: Sometimes Divine influence descends on one who is prepared for it without intermediaries. The influence will descend in such abundance that also those unprepared will receive. This is the metaphor of the good oil of Shemen ha'Mishchah. It primarily goes on the head, which is the place prepared for it; the brain and Kelim of intellect and thought are there. Oil symbolizes understanding. It happens to descend onto Aharon's beard, even though the beard is long, that it descends on his garments. The Nimshal is the influence of Chachmah on Yisachar (the head, which is prepared for it). He received a great amount, and it influenced also Zevulun.

Radak: When Moshe poured the oil on Aharon's head, it descended onto his beard and garments; he dressed him in the Bigdei Kehunah before pouring the oil. He anointed him between the eyebrows in the form of a Greek Chaf ¹ , and the oil poured on his head descended onto his beard and Midosav (garments, like "Mido Vad" - Vayikra 6:3). Pi (the head) of the garment is the neck opening. The oil was scented with a combination of fragrances, good and pleasant.

Rashi: From his head to his beard to the top of his Kesones (cloak); the beard rests on the Kesones. So is pleasant the dew of Chermon that is higher than the mountains of Tziyon (3); the dew descends from Chermon to the mountains of Tziyon. Just like Shemen ha'Mishchah is for grandeur, so Har Tziyon is for glory and honor of Yisrael (3). It is more blessed than all dew that descends in the world.