Allah (swt) swears by two types of trees: The fig and the olive, because of their great benefit to humans. Allah (swt) mentions the olive in more than one place in His Book. Allah (swt) says:

 {وَشَجَرَةً تَخْرُجُ مِن طُورِ سَيْنَاءَ تَنبُتُ بِالدُّهْنِ وَصِبْغٍ لِّلْآكِلِينَ}

(And [We brought forth] a tree issuing from Mount Sinai which produces oil and food for those who eat.) (Al-Muminoon: 20)

 اللَّهُ نُورُ السَّمَاوَاتِ وَالْأَرْضِ ۚ مَثَلُ نُورِهِ كَمِشْكَاةٍ فِيهَا مِصْبَاحٌ ۖ الْمِصْبَاحُ فِي زُجَاجَةٍ ۖ الزُّجَاجَةُ كَأَنَّهَا كَوْكَبٌ دُرِّيٌّ يُوقَدُ مِن شَجَرَةٍ مُّبَارَكَةٍ زَيْتُونَةٍ لَّا شَرْقِيَّةٍ وَلَا غَرْبِيَّةٍ يَكَادُ زَيْتُهَا يُضِيءُ وَلَوْ لَمْ تَمْسَسْهُ نَارٌ ۚ نُّورٌ عَلَىٰ نُورٍ ۗ يَهْدِي اللَّهُ لِنُورِهِ مَن يَشَاءُ ۚ وَيَضْرِبُ اللَّهُ الْأَمْثَالَ لِلنَّاسِ ۗ وَاللَّهُ بِكُلِّ شَيْءٍ عَلِيمٌ

(Allah is the Light of the heavens and the earth. The example of His light is like a niche within which is a lamp, the lamp is within glass, the glass as if it were a pearly [white] star lit from [the oil of] a blessed olive tree, neither of the east nor of the west, whose oil would almost glow even if untouched by fire. Light upon light. Allah guides to His light whom He wills. And Allah presents examples for the people, and Allah is Knowing of all things.) (An-Nour: 35)

It is an ascension of lighting a lamp from an olive oil tree, not a normal tree; neither of the east, nor from the west, blessed, of the highest purity oil comes out of it, which would almost glow even if untouched by fire, as if light upon light, there is not any point of darkness in this small circle. Such as Allah’s (swt) lighting of this vast universe, there is no point of darkness, rather, the whole universe is covered with Allah’s (light).

The oath here by the fig and the olive, not just because they are two plants or trees that people benefit from, rather, they remind us of the place where Muses received the message at Mount Sina’ where the Torah was descended and the place where our Messenger (saws) received the message in Makkah, the sacred city.