OpenAI's o-series represents a fundamental shift in how AI models solve problems. Instead of generating answers immediately, these models think first — producing an internal chain of thought before responding.
The Core Idea
Standard models (GPT-4o): Prompt → Answer Reasoning models (o1/o3): Prompt → Think → Think → Think → Answer
This extra thinking time dramatically improves accuracy on hard problems.
The o-Series Family
| Model | Released | Strength | Cost (Input/Output per M tokens) | |-------|----------|----------|----------------------------------| | o1-preview | Sep 2024 | First reasoning model | $15 / $60 | | o1 | Dec 2024 | Improved reasoning | $15 / $60 | | o1-mini | Sep 2024 | Fast reasoning | $3 / $12 | | o3 | Early 2025 | Best reasoning | $10 / $40 | | o3-mini | Early 2025 | Cost-effective reasoning | $1.10 / $4.40 | | o4-mini | Mid 2025 | Latest efficient reasoner | $1.10 / $4.40 |
When Reasoning Models Shine
- Mathematics — Competition-level math problems
- Science — Physics, chemistry, biology reasoning
- Coding — Complex algorithms, debugging, architecture
- Logic — Puzzles, constraint satisfaction, planning
- Analysis — Multi-factor decisions, risk assessment
- Legal/Medical — Interpreting complex regulations or symptoms
Benchmark Performance
- AIME 2024 (Math Competition): o3 scores 96.7% vs GPT-4o at 63.6%
- Codeforces (Competitive Programming): o3 reaches 99th percentile
- GPQA Diamond (PhD-level Science): o3 scores 87.7% vs GPT-4o at 53.6%
- ARC-AGI (Reasoning): o3 scores 87.5% vs GPT-4o at 5%
When NOT to Use Reasoning Models
❌ Simple Q&A and factual lookups ❌ Creative writing and brainstorming ❌ Translation and summarization ❌ Casual conversation ❌ High-volume, low-complexity tasks
For these, GPT-4o or GPT-4o-mini is faster, cheaper, and often better.
Cost Considerations
Reasoning models generate "thinking tokens" that you pay for but don't see: • A simple question might use 500 thinking tokens • A hard math problem might use 10,000+ thinking tokens • These hidden tokens significantly increase costs • Monitor usage carefully in production applications