Complex sentence: As deputy head of the counter-espionage bureau ("Abwehr"), Oster was in a strong position to conduct resistance operations under the guise of intelligence work; he was dismissed for helping Jews to avoid arrest.
Split into simpler sentences: As deputy head of the counter-espionage bureau ("Abwehr"), Oster was in a strong position to conduct resistance operations under the guise of intelligence work. But he was dismissed for helping Jews to avoid arrest.