ONN4ARG is an Ontology-aware Neural Network model for Antibiotic Resistance Gene (ARG) annotation predictions. It employs a novel ontology-aware layer to encourage annotation predictions satisfying the ontology rules (i.e., the ontology tree structure). Given a query protein sequence (either a manually entered sequence or an uploaded sequence file in FASTA format), the ONN4ARG server predicts the multi-level ARG annotations of the query protein in seconds.