"What had been a Quidditch match turned swiftly into a human versus tree battle...." (Pm)
The Order of Merit in Britain is an honorific award given at the discretion of the reigning sovereign (Wikipedia).
We don't know what ranking or relationship is between the International Wizarding Order of Merit and the Order of Merlin.
From the Web
Writing by J.K. Rowling on Pottermore: History of the Quidditch World Cup
Screenshots of the original Daily Prophet coverage of the 2014 Quidditch World Cup on Pottermore: Imgur archive