The .osk format is a skin archive where you can either double click it (if osu! is associated with the format) or drag it to your osu! window so the client itself will take care of the extraction and stuff it'll do in the Skins folder.
If you wanted a manual extraction, change the ".osk" format to ".zip" and extract the files in a folder then move the folder inside the Skins folder of osu!.
The Skins folder should look like the format below.
osu! folder
--- Skins (folder)
------- MySkin1 (folder)
----------- Skin files...
----------- Skin files...
------- MySkin2 (folder)
----------- Skin files...
----------- Skin files...
If you have osu! open, you need to restart the game client because it doesn't refresh the skins list.
If you have a resolution above 1366x768, it'll use the HD elements (@2x) first. If no HD element found, SD will be used.
Else, the default skin templates are used.