only Oath of devotion on paladin character sheet

is there a way to add the other Oaths for the paladin in the charactermancer? only has  devotion and no custom drop down to add the others.

If you don't own other content on Roll20, the charactermancer will only include content from the SRD.  You can't add to the charactermancer, but you could just make your character manually to include any physical print content you own, or edit the character after using the charactermancer to create it.  If you will be making a lot of changes, you might as well skip the 'mancer (in my opinion anyway).

so if i had the player's handbook it would allow me to use all the other stuff like the backgrounds as well?

Yes, that adds the PH content to the charactermancer and compendium.

brilliant thank you :)

If you are a player, ask if your game's creator (not always the GM) has it and will enable compendium sharing in your game. This will let you have access to his books purchased on Roll20.

I'm the DM i have the physical books so didn't bother with the roll20 versions but seems i have to get them really !

You don't have to. As kraynic mentions, you can just manual edit the sheet after you create it or just manually create it completely. There is nothing to force, or limit you in anyway, to purchase it.