Microsoft Corp. (Microsoft (MSFT)) ended fiscal 2026 with a record $678 billion in commercial remaining performance obligations (RPO), an 84% jump from a year earlier. But one comment from CFO Amy Hood during the earnings call suggests Microsoft's biggest AI customer may account for a surprisingly large share of that order book.

The company never disclosed OpenAI's backlog outright. Instead, Hood told analysts that commercial RPO "increased 25% when excluding OpenAI." Combined with Microsoft's prior-year disclosures, that comment offers investors a way to estimate just how significant the ChatGPT maker has become to Microsoft's future revenue pipeline.

If the estimate is directionally correct, it means roughly one out of every three dollars in Microsoft's contracted commercial revenue pipeline is tied to a single customer. That's an unusually high level of concentration for a company of Microsoft's size, even as the company works to diversify its AI business.

The Math Behind Microsoft's OpenAI Exposure

Microsoft reported commercial RPO of $368 billion at the end of fiscal 2025. If that backlog grew 25% excluding OpenAI, as Hood said, Microsoft's commercial RPO without its AI partner would now stand at roughly $460 billion.

That leaves an implied $218 billion difference between Microsoft's reported $678 billion backlog and the estimated $460 billion excluding OpenAI. In other words, OpenAI-related commitments could represent about 32% of Microsoft's commercial backlog.

This assumes OpenAI was a negligible part of the backlog a year ago. If it wasn't, the non-OpenAI base would be smaller—making OpenAI's share bigger than 32%, not smaller. Either way, one-third is a floor.

Microsoft did not disclose OpenAI's backlog or say one-third of its RPO belongs to the company. The figure is an estimate derived from Microsoft's reported RPO and Hood's disclosure about growth excluding OpenAI. Still, it highlights the extraordinary scale of Microsoft's commercial relationship with its biggest AI partner.

Microsoft Is Diversifying Beyond OpenAI

The disclosure came as Microsoft spent much of the earnings call emphasizing that its AI strategy extends well beyond OpenAI.

CEO Satya Nadella said Azure now offers more than 11,000 AI models, including offerings from OpenAI, Anthropic, Mistral, xAI and Microsoft's own MAI family. He also said the number of customers building applications with models from multiple providers has increased fivefold since the start of the year, underscoring Microsoft's push toward a model-agnostic AI platform.

Even so, OpenAI remains a major growth engine. Hood also said commercial bookings increased 18% year over year excluding OpenAI, while commercial RPO grew 25% on the same basis, illustrating the outsized contribution the AI startup continues to make.

Why It Matters For Investors

Microsoft has increasingly positioned Azure as the infrastructure layer for the broader AI ecosystem rather than a platform tied to any single model provider. But the company's latest disclosures suggest OpenAI remains one of the most strategically important customers in its history.

While the implied $218 billion figure is based on investor calculations rather than a company disclosure, it offers a fresh perspective on the scale of Microsoft's AI partnership—and why investors continue to watch that relationship as closely as Azure's headline growth.