Offensive Items:
The Static Shiv and the Infinity Edge give Yasuo all the crit chance he needs, crit damage, and most of his attack damage. The Bloodthirster gives him more attack damage and the lifesteal helps him to sustain, and helps him retain health during team fights. If the enemy composition is tanky, then you should switch Bloodthirster with the Blade Of The Ruined King. This is because of its passive which deals 8% of the targets current health, and does 10% of the enemies max health.

Defensive Items:
Spirit visage has a passive that increases your lifesteal ability which works great with either Blade Of The Ruined King, or Bloodthirster. It also gives you cooldown reduction, extra health, magic resistance, and health regeneration. Randuin's Omen gives you good armor, and gives you great health, but one of its most important parts is its active ability. If you ult a large group then you can hit Randuin's active which will make sure nobody is able to escape you after you are done ulting. Yasuo thrives in long team fights, but he needs to be able to survive long enough to get the stack of his Q and he needs enough health and armor to dive straight into the middle of the enemy team. If the enemy composition heavily leans towards ability power, then switch out Randuin's Omen for Banshees Vale. This gives you some added magic resistance but still gives you allot of extra health.

Summary: With this build you will be able to burst down enemies with tons of damage, while still being able to live long enough to dive into the enemy team, and get all your stacks of your Q.