If I wrote the following sentence using "ob" in the middle:
Kann ich dir das weiterleiten, damit du sehen kannst, ob der Text noch passt?
Does the definite article infront of "Text" need to be changed to den (for the accusative case) or can it remain der?
ob der Text noch passt is a subordinate subclause. der Text is the subject of that clause, hence it is Nominativ case. So, yes, your sentence is correct, and den Text would be wrong.
Short answer: No.
Ob, if used as a conjunction, works syntactically like other conjunctions (such as weil, warum etc.).
Like the other conjunctions, it asks for verb-last order (as in the example sentence) but there is no change of case.
Changing the order of clauses and words or slightly varying the meaning by using a different conjunction never changes a noun's case. It's the same in English: "I met him" vs. "It's him I met" or "I went to Berlin meeting him" vs. "I went to Berlin to meet him." The only difference is that German distinguishes between subordinating and coordinating conjunctions, but that only changes the word order, never the case.
