> how does OHDLC become HDLC?
OHDLC is designed to have many items share the same 'slot' and item code.
The most popular will get their own item code (and will be permanent HDLC).

> by putting OHDLC on a nintendo server?
No. The Nintendo servers give away items -- official Nintendo DLC (NDLC) and non-DLC (like flag patterns or built in items). They are active for approximately 2 weeks and country specific. Once Nintendo stops giving them away, you must get them by trading (or install them yourself with EZ_DLC_Install)

> if so, can anyone make HDLC available to the world?
Yes, once you install them (with EZ_DLC_Install) you can trade them with other non-hackers and 'infect' them with the new DLC.

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> also, is it possible to take the ISO of the game, edit items on the ISO...
Yes that is possible, but a lot of work for very little gain (IMHO). Your changes won't be seen by visitors.
Please see this thread for general graphic editing discussions: http://game-hackers.com/forum/showthread.php?t=1678

NOTE: the tools on this website (ACToolkit, NPC_Tool, PattView, EZ_DLC_Install) let you edit your town so that visitors will see the differences.