This build is for mid to late game. I made him OFF-TANK/AD. Great support if you have in team champions like Annie and Ashe. Why this build? Considering his hunger for mana, using his spells to spam a lot gets really fun and necessary so I chose a big mana build including a lot of health and with the help of Atma's Impaler things get dirty late game as you gain AD due to your health.

Pros:

- Very good farmer
- Good support for team
- Good warder and/or jungler
- Good solo laner in case you have a jungler

Cons:

- Early game Yorick is a bit squishy
- If you go for maximum damage on Yorick you are totally squishy late game and so you have to balance damage and armour
- RIOT still works at him and I'm sure he'll get better with his Passive and his Minions :)

First build:

Manamune - little ad but a lot of mana to help spamming especially with Omen of Pestilence.

Mercury's Treads - magic resistance + tenacity 35%.

Warmong's Armor - a lot of health + Yorick is a good farmer and you'll get max stacks in no time.

Atma's Impaler - more health = more damage + you get decent armour points (+45)

Banshee's Veil - blocks one negative spells which is very good for escaping or when ganking and also, you have Mercury's Treads and works quite well for an escape - gank route. For example Ashe can arrow you then tries to slow you: arrow disables the shield, slow effect is not so good with the help of the boots and you can use Omen of Pestilence which helps your allies to finish her off.

Frozen Heart - I chose this item because you get nice armour (+99), mana (+500) and CDR (+20%) which works well in spamming your abilities.

Oracle's Elixir - very good because you help a lot detecting wards and also you have some speed and resistance.

Second build:

Phage -> Frozen Mallet - good health + slow and with Atma's Impaler's bonus you gain a lot of damage.

Madred's Bloodrazor - you get 40% attack speed which works well with the amount of damage you do and also you get that lovely passive from this item.

Always Omen of Pestilence, you get slow plus a bit of damage and when you spam it a lot helps the team at most times. After OoP go for Omen of War, nice damage + that speed buff you get which will help you spam Omen of Pestilence. Omen of Famine is just a big Riot Fail, it does not heal you a lot but sometimes it might help you not die or steal time until your allies come and gank and also survive in gank.
Omen of Death - I usually use it on Singed for example, makes us 6 and helps ganking a lot if you can handle the pet actions.

Ghost - good for OoP and/or escape route.
Exhaust - support or kill.
Ignite - quite useful when picking on runners.

I'm not so into the other summoner's spells so feel free to experience.

It's very important that you stay in ganks and use your ulti, usually on tanks but in case other allies which are squishy are almost dead better use it especially on high DPS damange like Ashe.
Spam OoP always on low health champions especially on the melee ones.
OoW - use it on a minion/monster to gain speed and save allies.
OoP - good defensive when staying near the tower with low health (but risky) waiting for your allies to gank.

Although Yorick is a big fail from RIOT you can still be a big time support for your time with the spamming of OoP, OoW and OoD. Wards are the key of success in every game and also Oracle's Elixir for disabling the enemy warding.

More mana -> more spamming + more health -> more damage -> good results.

I know it's a hard build sometimes depending on your team but any suggestions would be nice :). Thank you and I hope you enjoy this build.