It's obviously hard to know, but hopefully it means he is responding well to the chemotherapy. Failure to respond to neoadjuvant chemo (chemo given before local treatment (surgery or radiation)) is a very poor prognostic sign for sarcomas.
Another issue is size and location of tumor - those with axial primary tumors (ie, pelvis, rib, spine, scapula, skull, clavicle, sternum) have a worse treatment outcome than those with extremity lesions. As far as I know that information has not been released (and I wouldn't think it would be).
Older patients (in this type of cancer that means >15 years old) historically did poorer, but recent studies have indicted this is no longer true (likely related to using different treatment in adults vs kids before, but now get the same).
One thing to remember in regards to his being ready to play next season, the treatment for this is quite long. While there are different regimens, the standard treatment is 7 cycles of chemotherapy given every 3 weeks (can also be done every 2 weeks), followed by surgery (if not feasible - ie disease in location or large enough that surgery would disrupt the function of an organ or be too destructive for a limb (amputation used to be quite common for sarcomas - see Terry Fox, although that was a different type) then radiation is the local treatment, followed by 7 further cycles of chemotherapy.
All in all treatment alone can take up to a year, especially if it is radiation over surgery. And there is absolutely no way that Oskar could keep himself remotely ready for professional hockey while undergoing this. Even with the best possible support team which I am sure he has, this takes quite a toll on the body. Most chemotherapy regimens, which are milder than used in sarcoma, take patients easily half a year to feel back to normal and often their baseline afterwards is weaker. A common saying is chemotherapy will automatically age you 10 years. Obviously for a typical cancer patient who is older, this can be a big deal. Oskar has the advantage of personal trainers/dieticians/etc so he does have better chances of returning to the level he was at before diagnosis. But I would be surprised if he is ready anytime this calendar year. I would expect at the earliest would be a year from now. But in these cases I am happy when I am wrong and expectations are exceeded.