Maybe Xerath is a OP Champion ;). He's one (or maybe "the") Best mid line Champion.
His damage is amazing combined with his long range, you are very strong and if you do it right you propably can't die and with ur stun combo you can easy dodge a gank.

It is up to u witch runes you use. I recommand Greater Mark of Insight for a well Magic penetration base, Greater Seal of Clarity for a nice late game mana regeneration, Greater Glyph of Force to push up you're late game ability power and Greater Quintessence of Pontency for a nice early game apility power base.

Xerath's farming is very strong. With your q attack you can easly get the lasthits by Minions. The q attack combined with your w attack is a good farm Minions and at the same time damage the enemy Champion Combo.

In the early game it is very nice to hold ur w attack on cooldown to damage the other champion as often as it is possible. If you can't kill him before level 6 then you can it easly with level 6. It is simple, damage him untill he's on half life (it is possible with the most midline champions), now start with a deadly combo first press w then do the e attack and stun him with your ulti then hit him with q (easy because he is stuned) then r and again r and death.

1vs1 is very simple you just have to do your deadly basic combo --> (w) e r(to easly get the stun) q r r and then if you have to, again (w) e q or only w and q for a deadly skillshot.

First, damage as often as it is possible with w and q the enemy champions befor the teamfight started. When the Teamfight started it is important to hit as many champions as possible with ur ulti. Look that you stun the champion who does the highest damage. Finish kills with w q skillshot combos if they run away, i want to call this ks (killsave).

Mejai's Soulstealer
If you notice the enemy team will gank you often or you die a lot then don't buy this Item.

Deffensive Items
If u have a strong enemy ap group or a feeded ap champion i will buy Abbyssal Scepter.
If you have a strong enemy ad group or a feeded ad champion i will buy Frozen Heart.
I swap one of these deffensive items with Mejai's Soulstealer in the early game and in the late game i will swap one of these with Morello's Evil Tome.