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You can switch rekordbox OSC samplers in the same way as usual samples.
1. Show sampler panel
2. Click OSC Sampler button to show the OSC sampler.

3. In the 4 OSC smapler slots, NOISE/SINEWAVE/SIREN/HORN are preset.
When you hover the slot you want to switch, the Eject button will appear.
Please click the Eject button to empty the slot.
4. Drag & Drop the sample you want to add to the empty slot.
When you want to return to the preset sample, find Sampler in the tree view and click the triangle before Sampler and click OSC SAMPLER > PRESET ONESHOT.
You can find the preset samplers there. Drag and drop them in the same way you did for other samples.
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You can select either the OSC sampler in DDJ-RZ or rekordbox dj.
If you want to use rekordbox OSC sampler, select [USB-A] or [USB-B], depending on which the cable is connected, from the [SOURCE] switch in the sampler section of DDJ-RZ.
