OPT is a 175 billion parameter open-source language model developed by Meta (Facebook) matching GPT-3's size, trained on 180B tokens with published training dynamics and logbook documentation — released to accelerate research on LLM interpretability, risks, and responsible deployment by providing the research community access to a frontier-class model without relying on proprietary APIs, and pioneering the transparent AI release model later adopted by many organizations.

Open Science Commitment

OPT distinguished itself through unprecedented transparency:

Scale Matching: OPT-175B achieved comparable capability to GPT-3-175B on major benchmarks despite different training approaches—proving that multiple paths lead to frontier performance and that scale matters less than community care in development.

Research Impact: The detailed training logs enabled breakthrough research on loss landscapes, emergent capabilities, and when behaviors emerge during training—answering fundamental questions about how LLMs learn.

Limitations & Growth: Meta transparently documented OPT's limitations (toxic outputs, lesser reasoning than ChatGPT)—pioneering "responsible release" practices that balance openness with acknowledging risks.

Legacy: Established that open releases of frontier models are feasible—security-through-obscurity isn't necessary, transparency builds trust, and research community responsibly handles powerful tools.