Question: Why the letter O shows up in this PDE solution?

What is the letter O doing inside this pde solution by Maple 2020.2 and what does it mean? There is no O in the PDE itself and it is not a name of a mathematical special function 

restart;
interface(version);
pde :=  diff(w(x,y),x)+ (a*x^2*y+b*y^3)*diff(w(x,y),y) = 0;
sol:=pdsolve(pde,w(x,y));

 

lprint(sol)

w(x,y) = _F1(1/9*(2*2^(2/3)*3^(5/6)*b*x*O*y^2-3*2^(2/3)*3^(1/3)*b*x*GAMMA(1/3,-\
2/3*x^3*a)*GAMMA(2/3)*y^2+9*O*GAMMA(2/3)*exp(2/3*x^3*a))/O/GAMMA(2/3)/y^2)

 

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