What is the significance of the words "always" and "okay" to the characters?

What is the significance of the words "always" and "okay" to the characters and why do they say ”okay will be our always”?

In film Isaac and his gf used to say always repeatedly and when Hazel and Gus didn't want to cut the call and said "Okay so many times, then Gus said OKay will be ours Always. Found nothing about "Always" from writer's perspective, maybe it was added just for the counterpart for okay or maybe as a sarcasm on a failed couple who used always but didn't last long.

Why didn't the writer choose another word instead of "okay" and "always"?

Film/book was inspired by the real-life story of Esther, and Esther used ok to her sister:

‘In the eulogy at Esther’s funeral, her sister Abigail references a conversation she had with Esther, when Esther was telling her, ‘you are going to be OK’.

‘OK became their word and that becomes the special word between Hazel Grace and Gus in the book and the movie.

Augustus's character was fictional and Esther didn't get someone like Gus:

In Esther’s journal she says, "Oh, I may never get to kiss a boy". She was ready for romance. She wasn’t looking for a knight in shining armour; she was more looking for someone with purpose, who was striving for something. Gus was ambitious and Esther would have liked that.