We would all die, there is no way for us as a species to replace oxygen as fast as we use it with out current level of technology and reliance on fossil fuels.
Think about it like this... we have roughly 80% of the worlds energy as fossil fuels and generated via burning. Coal, Gas and Oil account for basically the entire 80%. Renewables only form around 10% with the large majority being hydro... not solar or wind.. HYDRO. The rest of renewables is roughly 2%. (https://en.wikipedia.org/wiki/World_energy_consumption) (I leave out nuclear because as always, fear mongering has basically put it into the trash can and the plants take way too long to build).
So not only are we going to need to basically increase our current renewable production, we need to do it by 5000% in a single year because we run out of oxygen. (We need this energy production because without it, we dont have the power to convert Carbon dioxide into oxygen or water into oxygen.) (I also ignore hydro because that is very dependent on the terrain, water availability and requires the construction of huge dams to properly harness)
The problem with renewables and current renewable production and placement is that we have no way of doing anything significant in so short a time frame. We are ramping up production right now, but when that 1 year ticks by, oxygen levels start to drop, everything relying on power stops working because all our energy production methods rely on oxygen. What ever oxygen production plants we do have quickly become over crowded and people starve to death, because there is no way for them to get food (cars run on combustion, your kidding yourself if you think we can increase ALL current renewables by 5000% while converting all cars into electric and building oxygen conversion centers in a single year). Its not about renewables not being able to replace fossil fuels, its just we can't physically produce the numbers necessary to replace fossil fuels and our reliance on them.
Edit: just realised you put years.. I doubt we would have enough production capabilities even after 10 years. Not with our population still growing and the amount of time it takes for governments to understand the consequences that scientists have been telling us for over a decade and a clear lack of funding and push. Heck we probably won't even realize plants stop producing oxygen until a year later.
Edit2: I know 10 years is short, and I might be underestimating our supply (But seriously, its not going to be millions of years, maths below). The bigger issue wouldn't be the oxygen we breath but the lack of CO2 absorption by plants (since you take CO2 and create glucose and O2). That combined with all the algae in the world suddenly not producing oxygen would kill off sea life.
Okay and here is the maths I did with the non google sources (I got lazy okay).
So wikipedia says the atmosphere is 5.15*10^18 and 20.95% is oxygen which is 10^18 Kg of oxygen. (plenty)
NASA says we need 0.84Kg per person and with 7 billion people it doesn't even get close to the ball park of being a worry (Another answer did this). https://www.nasa.gov/pdf/146558main_RecyclingEDA%28final%29%204_10_06.pdf
This article https://www.sciencedirect.com/science/article/pii/S209592731830375X is a bit better and goes into oxygen consumption in different industries. Basically they estimate a 0.1% drop by 2100, so I assume 0.1% per 100 years. Still plenty of time
The issue is we can't go to 0% oxygen. We breath in 20%, breath out 15% so we need 5%. Americas OSHA https://www.osha.gov/pls/oshaweb/owadisp.show_document?p_id=25743&p_table=INTERPRETATIONS says an environment is oxygen deficient at 19.5%
so instead of having hunders of thousands of years for oxygen to drop to 0 we can only drop a single 1% before we are affected by it. Basically 1000 years. (16% and your out of breath while sitting, we if you want to push it, we can do the 5% drop). Still, thats plenty of oxygen, but no where near the figure that you guys have been saying. (To go on, 10% oxygen causes vomiting and 6% causes convolutions so 16% is probably the final limit we can hit before we need to recover).
The final pressing issue is that there is no oxygen renewal. Every single plant stops photosynthesizing. I assume they keep growing magically, but no oxygen comes out (and no oxygen goes in at night). Google said a tree produces 260 pounds of oxygen a year or 120Kg. Thats 120Kg per tree per year that isn't being produced anymore. Google also gives an estimate of 3 trillion trees, and that ends with roughly 2*10^14 Kgs of oxygen that doesn't get renewed per year and is removed from the cycle (I'm not sure if the science article references this or not). So we effectively lose 2*10^14 Kgs of oxygen as well as what we spend (I could be wrong in adding these... I didn't read the article in depth, only browsed through it).
And the final nail in the coffin is that algae, which produces 50%-85% of all oxygen (google again) stops. So at best, we lose 4*10^14 Kg of oxygen or 10^15 Kg of oxygen per year (Which is 25 to 10 years due to the 1% drop requirement). Algae also replenishes a lot of the oxygen in the sea, which is going to experience a sharp drop in oxygen levels and a huge increase in CO2 levels which will kill off that ecosystem (Algae wont be using that CO2 for the same reason plants don't. They get to absorb some to grow, but I assume the exact reaction to make oxygen no longer occurs).