The can be calculated by:
$%Yield=("Actual Yield")/("Theoretical Yield")xx100%$
The actual yield of oxygen is given and it is $10.2g$.
We will need to find the theoretical yield.
The decomposition reaction of water can be written as:
$2H_2O(l)->2H_2(g)+O_2(g)$
To find the theoretical yield of oxygen we will use dimensional analysis:
$?g O_2=underbrace(17.0gH_2Oxx(1molH_2O)/(18.0gH_2O))_(color(blue)("g to mol"))xxunderbrace((1molO_2)/(2molH_2O))_color(green)("molar ratio")xxunderbrace((32.0gO_2)/(1molO_2))_(color(red)("mol to g"))=15.1gO_2$
Thus, the theoretical yield is equal to $15.1g$.
The percent yield is then: $%Yield=(10.2)/(15.1)xx100%=67.5%$