Mobility: Why does Olaf need to move faster out of fights? Swiftness: Ult makes them redundant. Sorcs: Lol. Mercs: Ult makes them redundant. Lucidity: He has many strong options that easily max his CDR. Tabi: Armour is always welcome, I prefer these. Zerks: They give AS, which synergizes with his passive and W.
Yeah, I'd say Tabi are better, but Zerks aren't exactly a bad choice if you're lacking in damage.