OpenAI o3

o3 is the last standing o-series reasoning model, now sunsetting — fine for legacy use but replaced by GPT-5.4/5.5 reasoning for anything new.

Best for

• Legacy reasoning workloads already on the o-series
• Tool-using, multi-step reasoning not yet migrated
• Reproducing prior o3-based evaluation pipelines

How it compares — and the India angle

• End-of-life: o3 is on a sunset path and o4-mini is already retired — do NOT start new projects on the o-series.
• GPT-5.4/5.5 now beat o3 on GPQA, AIME and SWE-bench while folding reasoning into the main line.
• Indian teams: migrating off o3 now avoids a forced cutover later in 2026.