If you consider that the quality of randomness can only be assessed in proportion to the size of the data sample, you'll realise that it's not really that hard to make random numbers. Depending of course on how many you want. You have to change mindset from today. One time pads were not (and should not be) used to encrypt a 6TB hard drive's worth of porn.
I think in the early days, WW1ish time, they were made by ladies randomly typing at typewriters. If a pad's page only contains 250 numbers, that can be fairly easily be bashed out on a manual keyboard. If different individuals produce different pages, that should even out unconscious patterns. I've read this somewhere but can't find a reference. But if you try it yourself, you'll see it's easy. And how can you disprove the randomness of 250 characters that were typed randomly on purpose? You can't as historical archives show.
Later they had electro-mechanical randomising devices relying on timing a switch's contact bounce which is fairly random, and produced numbers at 35 bits/s. The following is an extract:-
and the following is the Hewlett-Packard 522B counter instrument. You can see the random numbers on the neon display:-
Later during the SIGSALY days, vacuum tube diodes were used to create electrical noise which was digitised. The key material was then distributed on a phonographic record. Users could talk for 12 minutes per record.
And whist the British Post Office is not the NSA, there's ERNIE which is a Premium Bond picker-outer machine. He's moved on now to
solid state diodes photon counting, but ERNIE 1 (1956) did it with these valves:-
V1 (extreme left) is the noisy valve. Interestingly, we still use the same circuit today. We just swap the vacuum tubes for transistors.
I'm not aware of any security breaches of a OPT other than the breaking of the GEE German Diplomatic code during WW2. All of the documented instances like (Venona) relied upon duplicated key material, not poor entropy from the typist. The proof of this is that no OTPs prior to the pad duplication mistake in 1942 have been decrypted.