Learn Dutch: Wij hebben olie nodig om de pitten in te roosteren.

This structure acts like an adjective describing the noun olie (oil). When you want to say "oil to roast the seeds in" in Dutch, you use om + [object] + [preposition] + te + [infinitive]. The preposition in gets "stranded" near the end of the clause rather than attaching directly to olie.

Why is the preposition in placed after de pitten instead of before it?

The preposition in relates to the oil (roasting in the oil), not the seeds. If you wrote om in de pitten te roosteren, it would mean you are roasting something inside the seeds! Because in refers back to the noun outside the clause (olie), it is separated from it and placed right before the te + infinitive.

Why does nodig come before the om clause?

nodig hebben (to need) acts like a separable verb, meaning nodig gets sent to the end of the main clause. The om ... te clause is an entirely separate infinitive clause. In Dutch, these longer subordinate clauses are placed after the main clause is already complete, which is why it appears after nodig.

I learned the word zaden for seeds. Why does this sentence use pitten?