To get mushroom oil in Tribes of Midgard, you will need to locate Stormcap Mushrooms and gather them in the Land of Pools biome in Tribes of Midgard.
You can find the Land of Pools biome, usually right next to the Blightforest biome or by just following the road to a certain location.
Accepting a quest that lists the Land of Pools biome as the quest location is also a decent way of finding the biome.
Related: How does Survival Mode in Tribes of Midgard work?
But before you try to find the biome, make sure you have at least starting weapons and armor, or you will find it a bit tricky to survive.
You should also consider making void or dark weapons as fast as possible as most enemies in the Land of Pools are weak to that damage type.
Also, try to visit the biome at night as Stormcap Mushrooms will turn into Glowing Stormcap Mushrooms and will give Moon Mushroom Bolts as an extra drop.
You will need these Moon Mushroom Bolts for a lot of recipes, so looting the Stormcaps is highly recommended.
But go gathering at night, unless you have a friend watching the village, as you can get overwhelmed by helthings quickly if you are not careful.
Check out some more Tribes of Midgard guides at Pro Game Guides!