BTS have the best selling single and album for a Korean group on the Oricon year-end charts, charting at #6 and #5 respectively on the top 50 singles chart and the top 50 albums chart.
TWICE are the best selling Korean group in terms of overall sales, ranking at #4 on the artists sales chart. TWICE also have multiple entries in both the top 50 singles chart and the top 50 albums chart, with more than half of the entries for Korean groups in the top 50 albums chart being from them!
Other Korean groups that made an impact on the year-end charts include: SEVENTEEN, IZ*ONE, Monsta X, and TVXQ.
Japan is considered to have the second biggest music market in the world (ranking only behind the US according to IFPI 2017 data).
Congrats to them.... Love them both