Flax (Linum usitatissimum L.) is a globally important oilseed crop, valued for its edible and industrial uses. Flax seeds are rich in unsaturated fatty acids. In this study, ethyl methyl sulfone was employed to construct a mutant library from the flax cultivar Longya 10 (WT). Screening efforts identified M45, a stable mutant with an oleic acid content of 43.22% at 40 days after flowering, representing a 21.23% increase over the wild-type. RNA-Seq analysis revealed the presence of two homologs of the SAD (stearoyl-ACP desaturase) family and two homologs of the FAD2 (fatty acid desaturase 2) family, which showed differential expression in a trend consistent with the phenotype of M45. A BSA-Seq analysis was conducted to identify genes with SNPs (single nucleotide polymorphisms) and Indel (insertions/deletions) variant loci that were associated with increased oleic acid. The combination of BSA-Seq, RNA-Seq, and metabolomic analyses identified L.us.o.g.scaffold122.86, a gene that may be co-expressed with L.us.o.g.scaffold7.26 to affect oleic acid accumulation via FAD2.