What can be unlocked in GTA Online from single player?

There is nothing like this that exists in GTA Online.

The only thing you could potentially be thinking about is the GTA Online Story Mode, which takes your GTA Online character through a scripted Story Mode in which you will interact with the characters from the Single Player Story.

Besides this and some other characters doing a crossover between Single Player and Online, they do not affect each other in any way.

Source (for GTA Online Story Mode)

GTA Online includes its own Story Mode. Taking place months before the events of GTA 5 single-player, the story takes the player's Character through series of Missions and encounters with GTA 5 characters.