Answer is: mass of oxygen is 12,576 grams.
Missing chemical reaction: 2KClO₃ → 2KCl + 3O₂.
m(KCl) = 19,6 g.
n(KCl) = m(KCl) ÷ M(KCl).
n(KCl) = 19,6 g ÷ 74,55 g/mol.
n(KCl) = 0,262 mol.
From chemical reaction: n(KCl) : n(O₂) = 2 : 3.
n(O₂) = 3 · 0,262 mol ÷ 2.
n(O₂) = 0,393 mol.
m(O₂) = n(O₂) · M(O₂).
m(O₂) = 0,393 mol · 32 g/mol.
m(O₂) = 12,576 g.
