I have a 520 Oliver baler, haven't had any issues getting knotter parts as my baler is in the same family as the 620/720 which became the hesston 4XXX series. The parts I've been getting from either AGCO or "AgPartsFirst" are grey/white in color. I'm guessing they are actually hesston parts.
What I couldn't get was a slip clutch for the pto. I used one off a New Holland Super 67 baler. Bolted right up.
I also have a New Holland 68. The balers are stored/parked side by side, but I've not done a great deal of comparison between them. Knotters are both of the same design, but I couldn't tell you if they would actually interchange. I can say that you could just about get any part you need for an older New Holland baler if you have the $$$.
In the field the 520 is far superior in the amount of hay that you can put through the machine. The bales are of same length each time. Makes a nice bale when it's not having issues with the knotters (I have faith that I'll get it fixed one of these days... each failure is just an opportunity to try again). I bought the bale more to take to shows with my tractors. However, it has to earn its keep.
The NH 68 makes a nice bale, not everyone comes out the same length, but close, and it never misses. Only downside, it's picky about the volume of hay you feed it.
Go Oliver green... you might have to get creative if you have a break down, but thats part of the hobby of old iron.