Sorry, but you either misspelled something or this post is bullshit. 1200 True damage in 4 seconds is not possible. At max rank, Olaf has 8 seconds CD on his E, the only ability he has with true damage. I'll indulge you and assume he had 45% CDR and is able to use two E's in about 4 seconds.
E's base damage is 250. He needs 875 AD to deal 600 Damage with each E.
In an almost impossible scenario, he'd be building full Attack Speed and CDR and would be hitting you non-stop. In which case he'd be able to use 3 E's in about 4 seconds (a little more due to cast times and stuff). In which case he'd need 350 AD. Now this one seems more reasonable, but I doubt it's possible to cap AS, CDR and reach such a high amount of AD, specially with two pure MR items.
If you say he dealt 1200 damage overall, then it's possible. But that's mostly due to him catching axes again and again, to which there is plenty of counterplay.