Ostarion, The Wraith King, is a meele hero, I personally think that Wraith King is the most versatile and most flexible hero in Dota because of how simple but terrible this hero is. Or maybe because I am a Wraith King spammer.

WK items should be Strength and a lot of attack speed.Pair your items and use the situational items when mana breakers/magic users in the enemy team are used.
Using WK requires a Push-Farm and Hunt-Kill method,Since WK needs a lot of gold to build, a bounty every 5 minutes will be a big advantage to WK(or just use Hands of Midas) an his teammates,When hunting be careful because of the wards,specially the sentry ones when using the Shadow blade when hunting.

After Core Items, Build BKB and then go for Situationals that would help you very much with dealing with disables like stuns and roots.

Having an Xp and Gold PROBLEM?
1.) Build Midas
*Use on creeps that drop the smallest gold or use on smaller creeps
2.) Avoid Dying
3.) Gank, Kill, Stick to teammates and most importantly "Efficiency"
4.) Space Out, Don't Panic and Never go clashing without your Special Skill
5.) Use your 3rd Skill (Skeletons) efficiently
*Let your Skeletons Push the lane
*Only launch skeletons at 3-4 Charges

There are some heroes that Counters WK very well.

*Anti-Mage-Because of its mana break and manta build
but AM's mana break can be countered by BKB
COUNTERED MOSTLY BY SILENCING HEROES LIKE SILENCER AND DOOM

*Riki-A Natural Diffusal Blade Carrier,Invisible and Silencer,Riki is very hard to kill and also can give vision to enemies
to counter Riki,you'll have to risk buying Gem of True Sight or It would be easier When Bounty hunter is picked with its track ability.
EASILY COUNTERED BY BLOODSEEKER OR BOUNTY HUNTER

*Phantom Lancer-A Diffusal Builder like riki but easy to counter with a Battle Fury(depending on your dmg) and also BKB for the mana break.
In using WK you should experiment the items you will need in a clash or solo kills.
EASILY COUNTERED BY EARTHSHAKER

*Lion-With lion's ability to siphon mana, it would be hard to lane with him
but since it's abilty is mostly magical rush the BKB and lvl 20.

*Terrorblade-Terrorblade's SS which is Sunder takes your HP and your HP after reincarnation when you think you are going to kill him.
BE PREPARED FOR THE NUKE ATTACK IN LATE GAME
1.) Either you should build Linken Sphere or BKB (Since it is Magical)

*Outworld Devourer & Silencer-
Silencer and OD's Intel steal really ♥♥♥♥♥ up with your mana pool which reduces your skill cast and Reincarnation from triggering.

(AND HEROES THAT ARE KITING HEROES LIKE STORM SPIRIT)
-wk spammer.