Winter hasn’t even started yet! Where do you live? And why do you want to wait until Spring?

I would not wait until the Spring, given the situation as presented. Call some qualified roofing repair companies for estimates, but act on this, as soon as possible.

We are in Canada, Quebec. The house is roughly 30 years old and we purchased the property this fall. When the house was inspected, it was reported that there was less than 6 in of isolation and so we had the intention of waiting until next spring to add isolation and wanted to take advantage of that time to renovate/fix any weaknesses or problematic areas in the roof. The first inspection was done in warm and dry weather, and so we do not know if the problematic sheathings (OSB) were overlooked by the inspector or if the problem started during the fall after our purchase (we had a period of heavy rain).

We went up to check on the roof again, and I have attached pictures in this post.

1) What we had interpreted as rotten OSB sheathings may actually be very wet/soaked sheathings. When we look from outside, the OSB sheathing is slightly deformed/wavy. When we look at it from the attic, we see that the OSB sheathing is currently black (wet/soaked with water). The sheathings seems to be holding its integrity, but seems waterlogged and we suspect it has went through (and still is) wet and dry cycles causing it to deform. Also, some parts started delaminating (wooden chips falling). Since winter is beginning, we will received more water/rain/snow, so it is unlikely that this will dry.

2) What do seem rotten is a portion where two OSB sheathings are supposed to join. We suspect they have lifted and water seems to have infiltrated through that lifted portion. Overall, we have 1.5 m x 5 cm of OSB sheathing, between two wet rafters, that is completely black and flaking (we see bits of black flakes falling from it - you can see these black bits on the isolation when looking at the picture)

Overall, we suspect that water condensed and accumulated due to bad ventilation (we found isolation blocking the soffits on the problematic side of the house). For now, we have pushed down the isolation and created about 15 cm of air flow between the wet OSB sheathings and the isolation (which itself was partially wet as well).

What are your thoughts on this? We may not have any professional help until at least mid/end of january, with the confinement and holidays coming up.

I see that there is a "Dr. Energy Saver" in Quebec. You may want to contact them to check your attic's insulation (isolation). They will remove and treat any mold, remove old isolation and correctly insulate your attic, while first sealing any holes from electrical work, etc. which leaks energy. Your soffit areas will be addressed to preclude any future potential for blockages. Your home will be warmer in winter, as well, and they will provide you with photos and a detailed estimate, and warranty. I had the U.S. division service my home (zone 6a) last year, and am very happy with the finished project, which took two days.

While you most likely need a new roof with ice and water guard and significantly better underlayment (spray on, if available in your area), having your attic area serviced will provide you peace of mind, as well. I would strongly recommend you contact contractors for estimates, now, as well as to get on their schedules, early. Only these professionals can truly address your concerns, but I'd be sufficiently concerned as to call them, now.