A polymer, an organic layer composition including the polymer, and a method of forming a pattern using the organic layer composition, the polymer including a reaction product of a first compound represented by Chemical Formula 1 and a second compound represented by Chemical Formula 2: (CHO)n1—Ar1—X—Ar2—(CHO)n2  [Chemical Formula 1] wherein, in Chemical Formula 1, X is O, S, or NR, in which R is a hydrogen, or a substituted or unsubstituted C1 to C30 alkyl group, Ar1 and Ar2 are independently a substituted or unsubstituted C6 to C30 aromatic ring group, and n1 and n2 are independently an integer of 1 to 3; Ar3—(OH)m  [Chemical Formula 2] wherein, in Chemical Formula 2, Ar3 is a substituted or unsubstituted C10 to C30 fused aromatic ring group in which at least two rings are fused, and m is an integer of 1 to 3.