Head-to-head comparison
GPT-5.3-Codex (OpenAI) and o3 (OpenAI) compared on the ModelCap Index, API price, context window, provider availability, weight access and every public benchmark board they share. Figures are the same ones shown on the live rankings; nothing here is a hidden score.
Snapshot as of 17 August 2026
As of 17 August 2026, GPT-5.3-Codex holds the stronger ModelCap Index position (#20 vs #38); o3 is 1.8× cheaper per output token ($8.00/1M vs $14.00/1M); and GPT-5.3-Codex offers the longer context window (400K vs 200K tokens).
Evidence: 1 public benchmark observation across 1 board · 4 public benchmark observations across 4 boards. Prices are the lowest listed API offer per million tokens observed on the OpenRouter catalogue.
Scores are the sources' own published figures for each model's best evaluated configuration; ModelCap never re-runs a benchmark.
Want a different pairing? Open the interactive comparison tool to swap either model for any current ranked language model.
GPT-5.3-Codex ranks higher on the ModelCap Index as of 17 August 2026: #20 against #38. That is a capability ranking built from public benchmark evidence with published uncertainty; whether it is "better" for you also depends on price, context and where you can run it.
o3 is cheaper on output tokens: $8.00/1M against $14.00/1M. Input tokens are $1.75/1M for GPT-5.3-Codex and $2.00/1M for o3. Prices are the lowest listed API offer ModelCap observed, in USD per million tokens.
GPT-5.3-Codex has the larger context window: 400K tokens against 200K.
The two models do not share a coding benchmark board on ModelCap yet, so no head-to-head coding score is published; the ModelCap Index position is the closest overall signal.
GPT-5.3-Codex: API only. o3: API only. Open weights mean the checkpoint can be downloaded and self-hosted under its licence; API-only models are available solely through hosted endpoints.
ModelCap currently lists 2 API providers for GPT-5.3-Codex and 1 for o3, from the OpenRouter catalogue snapshot the site serves; each model page lists the providers and their prices.
Every figure comes from the sealed ModelCap dataset published 17 August 2026; the page re-renders within a minute of each data refresh, and the ModelCap Index positions are the same ones shown on the live rankings.