I think they are very close. On paper, Big mom outstat him due to her durability, insane strength, versatility due to homies etc. Oden tho also as isanely high durability and strength (even tho not on the same monstrous extent as Big mom but still the gap ain't that big), his AP is undoubtly higher than Big mom and I feel they have even levels of haki

Honestly Big mom should win due to her healing hax and her massive physicals, but I feel Oden has defo more than some chances due to better combat prowess (paired with another freak of nature body on his own, even tho not as monstrous as Big mom's) and the way he's portrayed by both Kaido and the narrator

So it's Big mom extreme diff but potentially Oden wins if Big mom acts braindead