Not sure if my solution will count as a true OP complete for the challenge solution and for the challenge but here it is anyways.
This week there is a challenge for the Maus http://xbox.worldoftanks.com/en/news/game-events/ontrack-mau...
simply play any one of the tanks in the Maus tree 10x doesn't matter win or lose it will count count as OP completed:
II Pz.Kpfw. 35 (t)
III Pz.Kpfw. 38 (t)
IV Pz.Kpfw. 38 (t) n.A.
V Pz.Kpfw. IV
VII Pz.Kpfw. VI Tiger (P)
VIII VK 45.02 (P) Ausf. A
IX VK 45.02 (P) Ausf. B
please note that this op ends in 16 days of this post:
Wargamming usually has ON Track ops every month so you can do this with what ever tank they make ON track and use any of the tank in that on track tree.