Olaf should be able to extend his ult if he gets a kill...

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Doesn't Olaf's MS buff only last for 1 second and only towards enemy champions where Master Yi just gets his for the entire duration?

It lasts 6 seconds. On top of additional MS and CC immunity, it also gives bonus AD to mirror Highlander's bonus AS, hence making the 2 abilities very comparable. It only gives MS when running towards visible enemy, sure, but it's not like Yi uses his ulti that much differently. Thus the fact Olaf was completely CC immune (unlike Yi who is only immune to slows during ulti) for the duration while actually running faster towards enemies make the ulti already arguably more appealing than Yi's on an overall scale.

The only perk Yi has over Olaf in this regard is that Yi's ulti can be used for slightly better roaming and have higher staying power (and that is IF he manage to kill people), so if Olaf somehow manage to get the on-kill extension too, it will make the ulti a flat out superior version of Yi's ulti.

.....

EDIT: MB, so it only lasts 1 second. But still, it's not like Olaf uses it at places where he can't chase the enemy down, especially when he also has a thing for Deadman's Plate, Frozen Mallet, or just his throwing axes. I still stand by the notion that he shouldn't get on-kill extension.