Everything I've ever read about the history of the Liberator has indicated in either direct, or indirect, terms that when the Liberators were officially turned over to OSS, Donovan and the rest of the high mucks in the organization A) didn't know to that point that they existed, and B) when they heard what they were originally conceived as, rolled their eyes and stuck them in storage.
I've no doubt that individuals in OSS knew that the gun/project existed, but given that it was primarily an Army conception, it's not at all surprising that the upper leadership of OSS didn't know about the program.
There were dozens, if hundreds (or more), of projects under way at any one time that had some form of OSS participation. It's pretty fanciful to think that every one of those projects would have been routinely briefed to OSS' upper leadership.
Given that the Liberator project officially kicked off in June 1942, or at exactly the same time that Donovan was thrust into the leadership position of a new organization that he had to get up and running in accordance with the plan that he presented to Roosevelt, I sort of thing he and his henchmen were busy with other things...
Once the Army showed up and said "Surprise, here's your 1 million weird assed guns," yeah, OSS leadership would notice.