Lay Waste is very strong at farming minions and harassing enemy champions.
Ask your allies to help point out when you should use Requiem to get kills in different lanes.
Try to stay away from walls. Ornn is much less strong if he cannot stun you.
Attack Ornn to keep him from creating items in lane.
The stat comparisons shown here feature several strategic Karthus against Ornn matchup stats that may help you interpret the differences and similarities between this pair of champs. Karthus’s KDA ratio ([kills + assists] / deaths) of NaN is better than Ornn’s KDA ratio of NaN, showing that Karthus may be more central to his team's team fighting effectiveness than Ornn. This observation is in large part fueled by the differences in deaths.
Karthus often has a slightly larger longest killing spree than his matchup does. On average, he receives less damage than Ornn. This commonly reflects different amounts of tankyness, yet it can also indicate that the one champion has less mobility and thus is not able to flee from additional damage when engaged or poked.
Both LoL champs are great. Both have their strengths, weaknesses, and counters. In the game's current meta, Karthus usually loses when taking on Ornn, with a 46.4% win rate. Thus, Karthus makes a bad counter for Ornn.
While Karthus does have a lower win rate than Ornn, when on opposite teams, Karthus also has a greater level of difficulty that makes him a more time consuming champion to develop with. You should expect to face a difficult match when picking Karthus into Ornnas his relative depth means you may not be able to unleash his full potential without a significant amount of practice beforehand.
Additionally, Karthus also has some amount of utility and CC, a similar amount to Ornn. This makes her just as valuable during teamfights, especially when trying to counter champions with a ton of burst damage.
While there isn't one best champion in League of Legends, in Karthus vs Ornn matchups, Ornn is the better champ with a much lower win rate, more champion depth, and a similar amount of utility to help out your team members during late stage team fights.
Karthus is a terribly counter for Ornn. You will do well by focussing your tactics on keeping up your CS and clearing objectives. If you do that, you should do very well as Karthus against Ornn.
To learn all of the intricacies of Karthus to counter Ornn during both the early game and mid / late game phases of League of Legends, you should continue reading to pick up a few extra tricks and insights into this matchup. If you mind the build and suggestions displayed here, you will grow your win rate significantly and be closer to League of Legends pro players.
Karthus often picks up less CS as Ornn. Champions who typically don't acquire many minion kills usualy don't need much CS to be valuable teammates, such as support champions. They are capable of scaling well off of their skills, stats, and first items alone. Yet, champions with a lot of CS, such as hyper-carries, often need several high cost items to be effective. In either situation, try to exceed the values shown on this page to do well.
The best items to prioritize in your Karthus versus Ornn build consist of Rabadon's Deathcap, Shadowflame, and Liandry's Torment. When Karthus used at least these three items in his build, he did a lot better countering Ornn than with most other typical builds. In fact, Karthus had an average win rate of 46.4% when countering Ornn with this build.
To have the highest probability of annihilating Ornn as Karthus, you should equip the First Strike, Magical Footwear, Future's Market, Cosmic Insight, Eyeball Collection, and Ultimate Hunter runes from the Inspiration and Domination rune sets. Out of all the runes players picked for Karthus vs Ornn counter picks, this combination of runes resulted in the best win rate. Notably, these runes gave a 46.4% winrate overall.
By default, tips, statistics, and builds on how to beat Ornn as Karthus are given for all ranked divisions combined. If you want to narrow the statistics and builds to a particular rank, you may use the selection menu earlier on the page.