Linked lists, as far as I have seen, are largely implemented using object-oriented ideas. (having an object that holds some information and the address of the next link).

What have you seen that is not object-oriented? If the only things you've seen are OO then it is not surprising that the only implementations of simple data structures you've seen are OO.

Were they only invented(?) once the OOP was developed?

Linked lists predate OO programming by many decades.

How were Linked-lists implemented before the object-oriented paradigm came out?

In the 1950's the Lisp programming language was implemented on the IBM 704. The fundamental data structure of Lisp is the cons cell, which is a grouping of two values. The machine word size of the IBM 704 was 36 bits and there were special instructions, CAR and CDR that would extract 15 bit values from a 36 bit word. The value stored in the CAR bits was the head of the list and the value stored in CDR was the tail, so in that way a cons cell could be used as a node in a linked list.

For a more detailed discussion of how linked lists were implemented on the IBM 704 in the 1950s, see http://en.wikipedia.org/wiki/CAR_and_CDR.