Oni has very low ATK for a 7G monster, but has a high HP stat of 9, making Oni a slightly weaker, Tauntless version of Greater Dog.
However, that won't stay forever. Because Oni's ability gives it +2 ATK at the start of your turn. This makes it a huge threat even after just 1 turn. And because it has 9 HP, it's not easy to kill it in 1 turn either.
You can also use a Faun/Toriel + Muffet's Pet combo to get rid of it. Just keep in mind that you shouldn't use Faun or Toriel by itself if Oni isn't silenced, as it's ATK would just raise back up when it's turn starts.