Pensieve Header: Experiments with “Collect”.

t1=Expand[Normal[Series[E^((a+b)x), {x, 0,5}]]] + ((O[x])^6)

t2=Expand[Normal[t1]]

t3=Collect[t2, x]

t4=Collect[t2, x, Simplify]

t5= t2 /. x^p_. :> x[p]

t6=Collect[t5, _x, Factor]