Olaf is the best champion in the game. His early game is absurd, and his late game is crazy good. Olaf is also the best snowballer, which means once he gets a kill, its gg.
This build capitalizes on olaf's snowball potential, by getting snowball items such as SotO and Leviathan, since your pretty much guaranteed to get kills unless you have downs or something.

Pros

He's olaf
He's a viking
best champion
kills dragons (#1 shyvana counter)
Best legendary skin in THE GAME. BETTER THAN GAYMAN CHOGATH
unkillable
snowballs like a boss
unstoppable
axe axe axe huehuehue
gives you a ****** when you smite someone down with your TRUE DAMAGE LIGHTING AXE
Can't be CC'd
can 1v5 easy np
can't die
IS A BRO
had sex with your mom last night

Cons (lol)

throat gets sore from yelling at screen all day
you get tired from winning so much and so hard
your team will go gay for you

Start with a ruby crystal, because olaf don't need damage items to WRECCKKK
build into leviathan, and get first blood
ALWAYS TAKE THE KILL.
this is very important.
on leviathan and sword of the occult, a kill gives you 2 stacks, while an assist gives you 1
WHO NEEDS IT MORE, YOU OR YOUR TEAMMATES
YOU BECAUSE YOU ARE OLAF AND YOU SNOWBALL AND 1v5 THEM AT THEIR FOUNTAIN AND WIN WITH FULL HP
YEAHHHHHHHHHHHHHH
Get sword of the occult asap and then fully stack both snowball items.
Take all the buffs from your jungler asap. WHO NEEDS IT MORE
YOU.
start farming like a boss when you get bored of instagibbing squishies and tanks with your TRUE DAMAGE
buy warmogs for EVEN MORE HEALTH AND MORE INVINCIBILITY
next get atmas for EVEN MORE DAMAGE AND EVEN MORE LIFESTEAL
next you can get another warmogs if you haven't won already
boots are optional, because you already have the 15% movement speed from fully stack sword of the occult, but i guess you can get boots of mobility if you want to look like a viking truck charging into a 1v5 and winning
If you die, /surrender and uninstall and throw your computer down the stairs because you are probably ******ed because olaf is the best and can't die
Alternative build:
6 Trinity forces.

GG

Get teleport and ghost.
Not for dragon control or escaping or any of that useless **** because you're too busy smiting people with your TRUE DAMAGEEEE and winning to give a care about 190 global gold or running like a coward noob STAY AND FIGHT YOU ***** YOUR GONNA WIN ANYWAYS DOESNT MATTER IF YOU HAVE 2 HP YOU'LL JUST LIFESTEAL ALL THAT **** RIGHT BACK YEAHHHHHHH
Get tele and ghost to get back to lane ASAP SO YOU CAN FARM THAT **** AND GET TRIPLE KILLS

NONE HAHAHAHAHAHAHUEHUEHUEHUE

max e first because true damage is OP
max axe second because no one can run from you
max w last because you really only need one point in it to 1v5 and win
get ult whenever

Get your items, go solo top
Get first blood with true damage, farm until you get leviathan.
Teleport back top, overextend like hell
"but Break, what if their jungler comes and they 2v1 me?"
to that i say:
WHAT ARE YOU A *****
YOU ARE OLAF
YOU DONT GIVE A **** ABOUT SOME WEAKASS JUNGLER AND THEIR WEAKASS SOLO TOP
TAKE BOTH THOSE *****ES ON WHILE LIFESTEALING WITH W AND CHUCKING AXES EVERYWHERE AND YELLING AT YOUR COMPUTER AND SMITING WITH YOUR LIGHTING AXE.
double kill.
YEAHHHHHH

Olaf is also the best jungler.
Just don't miss your axe
Really thats all there is to it

LOL WHAT TEAMWORK
YOU'RE OLAF
YOU ARE THE TEAM
make sure your teammates know that by c/p'ing it in chat every few minutes

In conclusion, olaf is the best, has no counters, and can win a game by himself.
He's really fun and you cant lose
HUEHUEHUE