# Truncating power series

I have the following typed up so as to truncate higher powers of $\tau$,

1. x^2 τ^1. - 2. x y τ^1.5 + y^2 τ^2. +
2. x^2 τ^2. - 3. x y τ^2.5 + O τ^3 +
1. y^2 τ^3 + 2. x^2 τ^3. - 2. x y τ^3.5 +
0.25 y^2 τ^4 + 1. x^2 τ^4. - 0.5 x y τ^4.5 +
0.25 x^2 τ^5. + O[τ]^3


Unfortunately, the big-oh notation does not give me results that I expect. What am I doing wrong?

The problem is floating-point powers. Rationalize solves the problem
Rationalize[