A loss of consciousness at work is recordable on the 300 Log, although you do not have to call it in to OSHA. However, reporting to OSHA is required for in-patient hospitalizations under 1904.39. If your employee was admitted to the hospital for a work-related event, you would need to contact OSHA within 24 hours.
The exception, of course, would be if the loss of consciousness was solely caused by a non-work condition that happened to surface at work (like diabetes). That’s one of the exceptions under 1904.5: The injury or illness involves signs or symptoms that surface at work but result solely from a non-work-related event or exposure that occurs outside the work environment. Such an event would not be work-related, and should not have to be called in to OSHA, nor would it be recordable on the 300 Log.
Finally, when a loss of consciousness is work-related, OSHA doesn’t describe how to record it, but does note that injuries result from a single event (like a cut or burn) while illnesses develop over time, even if only a few hours (like heat stress). In theory, a work-related loss of consciousness could result from a specific event (OSHA gives the example of passing out at the sight of blood) or it could result from a longer exposure (like passing out from heat stress).
These would be “other recordable cases” (if they do not result in days away, restriction, etc.). And if it resulted from a single event, which seems most likely, it would be an injury. But if it developed over time, it would go under the appropriate illness category. The most likely seem either “poisoning” (for exposure to gases, vapors, etc.) or “all other illnesses.” Information from the hospital or employee should help you identify the cause.