I'm not so sure that Oklahoma is Heupel's dream job. Stoops fired him and they've been cold ever since then. Oklahoma chose Lincoln Riley over Heupel. He seems to be genuinely peeved about all that. He wants to prove everyone wrong, just like he did when he was a college QB and major schools ignored him.
What could possibly prove everyone wrong more than restoring Tennessee to greatness? He'd basically been doing exactly what Stoops did at Oklahoma.
Besides, people forget that Heupel was only at OU for 1 year. He went to school at Weber State then Snow College. Think he was a 5th year player by the time he played at Oklahoma.