If you have Latin Modern fonts on your system you get a better result with
funs = {Sin[x], Normal[Sin[x] + O[x]^8]}; g =
Plot[Evaluate[funs], {x, 0, 2 Pi}, AxesLabel -> {"x", "Sin"},
PlotLegends -> {Style[TraditionalForm[Sin[x]],
FontFamily -> "Latin Modern Roman"],
Style[Normal[Sin[x] + O[x]^8],
FontFamily -> "Latin Modern Roman"]},
BaseStyle -> {FontSize -> 10, FontFamily -> "Latin Modern Roman"},
Frame -> True]
Otherwise try with the Times font family.
