Think about what you have here.
Your original mixture contains iron(II, III) oxide,
After you heat this mixture, it will react with the oxygen in the air to produce iron(III) oxide,
Notice that the mass of the mixture increases from
The amount of oxygen the react with the initial mixture to form the iron(III) oxide!
This means that the mass of oxygen that reacted will be equal to the difference between the final mass and the initial mass of the mixture
#m_(Fe_2O_3) = m_"mixture" + m_"oxygen"#
#m_"oxygen" = m_(Fe_2O_3) - m_"mixture"#
#m_"oxygen" = "4.141 g" - "3.869 g" = color(green)("0.272 g")#