I was doing great with my bonsai
It's a seedling, not a bonsai. It takes many years to grow a tree that's suitable for bonsai - say some 15 years to grow a pre-bonsai.
It is fun to grow trees from seed, but growing bonsai this way is not done often as it takes far too long (I am repeating myself... the same answers have been given recently, I think yesterday, and are given very regularly). For beginners, it practically never works out.
I had the bad idea to ask watering advices to the gardener of my office and he told me I should water it every 7-10 days
Not a good advice indeed, but also keeping it in an office - a bad climate for plants, very bad for outdoor trees, and a terrible climate for seedlings.
My office has 3 floors and and heating system is centralised and I wasn’t thinking they were turning on that week, and also with a equatioral temperature..
If you really want to grow an oak bonsai, get more mature material. Sure you can grow from seed as well, but grow it outdoors, where it belongs. And have lots of patience.