Oomycetes differ from true fungi because they have
Oomycetes historically misclassified as fungi due to filamentous growth and absorptive nutrition but belong to Stramenopiles heterokont lineage. Distinguishing biochemical trait is cell wall composition predominantly beta-glucans and cellulose synthesized via cellulose synthases, lacking chitin entirely which defines true fungal wall. True fungi walls contain chitin and beta-glucans. Additionally oomycete hyphae diploid, produce biflagellate heterokont zoospores with anterior tinsel and posterior whiplash flagella. Molecular phylogeny confirms convergent evolution of hyphal morphology, necessitating cellulose wall as key differentiating character separating Oomycota from Eumycota.
Ref: Campbell Biology 12th ed., Chapter 28: Oomycetes vs True Fungi Stramenopiles; NCERT Class 11 - Oomycota Cell Wall; NCBI