A strict gluten-free diet (GFD) can be diversified by non-contaminated oats, but there is a shortage of long-term studies concerning its safety. We compared long-term treatment outcomes and factors associated with the introduction of oats between celiac patients on a GFD with or without oats. Eight hundred sixty-nine previously diagnosed celiac patients were interviewed. The validated Gastrointestinal Symptom Rating Scale (GSRS), Psychological General Well-Being (PGWB), and Short-Form 36 Health Survey (SF-36) questionnaires were used to assess symptoms and quality of life, serological tests were performed, and results of histology were confirmed from patient records. We found the median duration of GFD to be 10 years and 82% using oats. Factors predicting the consumption of oats were diagnosis after the year 2000, advice from a dietitian, detection by screening, and mild clinical presentation. Oat consumers and non-consumers did not differ in dietary adherence (96.5% vs. 97.4%, p = 0.746), the prevalence of symptoms (22.9% vs. 22.5%, p = 0.931), positivity for endomysial antibodies (8.8% vs. 6.0%, p = 0.237), histological recovery after one year (63.1% vs. 60.0%, p = 0.773), malignancy (4.8% vs. 3.3%, p = 0.420), osteoporosis/osteopenia (9.2% vs. 11.0%, p = 0.489), or fractures (26.9% vs. 27.9%, p = 0.791). The oat consumers had better SF-36 physical role limitations and general health scores. Based on our results, the long-term consumption of oats in celiac disease patients is safe and may improve quality of life.