@GEPPETTOSMONSTER:
EDIT: btw does anyone know how well the first volume is selling?
Actually not too bad. It's not surprising the first Tank didn't make the Top 30 since new series are always printed in a small circulation as a matter of caution. (That is if they're not drawn by veterans [e.g. [I]Bakuman, Ane-doki, etc.] or JUMP decided to push the shits outta said series [e.g. [I]Kuroko, Beelzebub, etc.])
In Oumaga's case they miscalculated and OD Vol. 1 was out of stock at Japanese Amazon within 45 minutes. They refilled their stock twice and each time that didn't last a single hour.
On the cover of chapter 17 it said "selling and enjoying great popularity" and on this week's chapter's (no. 19) cover it is announced they'll launch an "urgent second printing due to the great popularity"!
I didn't find any concrete numbers yet but I have read at some other site specialized in Manga sales that OD's numbers were a little better than Kiben Gakuha Yotsuya-senpai no Kaidan which sold about 20,000 books with its first volume.
Now, even though this doesn't sound too reassuring seeing how Yotsuya's was canned eventually (despite being a culty nice series), we should be happy that at least some fuckers in Japan seem to appreciate this series as much as we do! Se, let's keep our fingers crossed their numbers will increase as well. :)