Received my one plus 2 but... ads are appearing

Hi everyone,
Just received my one plus 2 from oppomart.
Very nice phone and 100% compatible with EU carrier.
I've Android 5.1.1
Oxygen version: 2.0.0
And it's the 64gb version.
I've started it and it just downloaded all my apps with my google user exactly the same as my old samsung GS4.
But... I don't know why, I'm having sometimes some ads appearing! That makes me sick! I've a popup on the middle of the screen with different ads!
Does somebody know how I can get ride of this ? I've exactly the same apps as I had on my GS4... and on the one plus 2 I've those stupid popups appearing.
I've of course already restarted and reseted my phone but that keeps coming :crying:
Any idea for this scary problem ?

Thanks in advance.
Mourte