TLV’s level 13 Hunka Burnin Olaf has not ever really performed well, it had its moments back when zagara ruled the meta, but since then it has had virtually no uses. My suggestion to fixing it is to make the fire spread from olaf to other vikings (and from said vikings to other vikings).
My suggestion as to how this would work is to have any viking who enters burn range of any burning viking aquire the burn effect for 5 seconds AND, when olaf charges, the burn effect deals double damage and goes out 5x as far for it's next damage tick.