How do I unlock the hidden synth?

There is no hidden synth mentioned in the OP-1 manual. The OP-1 comes with a set of synthesizer engines that are all accessible through the standard interface.

Available synth engines

You can access all engines by pressing shift + T1 while in synthesizer mode. The available engines are:

There is also a synth sampler engine available when you select a sampler-based sound slot.

Switching engines

Press the Synthesizer key, select a sound with keys 1-8, then press shift + T1 to open the engine browser. Turn the blue encoder to scroll through the list and press T1 to confirm your selection.

If you heard about a hidden synth from a third party source, it may refer to a specific firmware version or community modification not covered in the official manual.