Omniknight as a core hero is often overlooked, as most people have the mindset of playing him as a position 4 or 5 hero. He does very poorly usually, as Omni has several weaknesses that need to be addressed by items and levels in order to really shine, namely his short cast range, bad mana pool, and squishiness without some items.
If played as a position 3 in the offlane, or very rarely as a position 1 with a carry like Weaver mid, Omniknight can easily become an insurmountable obstacle to the enemy team with his massive amount of sustain for his team and himself, as well as putting out far more damage than expected with Hammer of Purity and Purification.