Generate Blocks Mode
The Generate Blocks mode allows you to convert a Voxels object into a Blocks object.
To enter the mode you need to have a Voxels object in your scene. Select the object and in the Properties in the Actions section click the Generate Blocks button.

When you enter the mode, your Voxels object will automatically be converted into a preview Blocks object. When you make changes to the settings in this mode the preview Blocks will be updated.
When you are happy with the current preview object, click the "Apply" button to exit the mode with your new Voxels object.
You can move the Voxels object in this mode by using the 3D gizmo. The blocks will automatically rebuild when you are done.

Settings
Block Atlas
The block atlas determines what blocks are available to use and the textures look like. OtS has atlasses for the final version of each major update to Minecraft from 1.13 onwards.
You can create custom atlasses to include modded blocks, use resource packs, or use specific Minecraft versions such as April Fools updates. To learn more how to create custom atlasses visit the Custom Atlasses page.
All blocks in the 'Block Palette' section are blocks that will be used. All blocks in the 'Available Blocks' section will not be used.
When you upload a custom atlas you will have to exit and enter the mode before it is shown in the list of available atlasses. This will be fixed in the future.
Block Palette
The block palette determines which blocks should be used in the preview Blocks out of all the blocks available. This is extremely useful for removing blocks you don't like or want to build with.
For example, if you are building in survival mode then you will likely want to disable blocks like minecraft:bedrock
and minecraft:ancient_debris
.
Modifiers
Modifiers allow you to add additional modifications to the preview Voxels object. Modifiers are optional and are applied one after another.
There are various types of modifiers and you can apply them in any order and can apply multiple of the same type.
Replace
The Replace modifier replaces one type of block for another.

Script
The Script modifier is a Pro modifier that lets you upload a Lua script to programatically modify/add/remove blocks. For more details, visit the Lua Scripting.