A definitive time line would be impossible for OSD, I think. The games don't exactly have a story bible. And what's more, breaking continuity is encouraged (and almost required). But this is the OSDverse as I understand it:
1) Ben304 gets very drunk and makes Scourge of the Underworld. This is the beginning of something awesome and terrible in the AGS community. No one can go back to their normal, everyday lives after this moment.
2) DDQ, not to be outdone, breaks out his crayons and doodles over Ben's game, making the DX version. It is at this point that Life Partner Ray is introduced and announces that he's off to play Snakes of Avalon.
3) Ben returns with Mighty Pirate, in which Dennis tries to get Ray's attention by starring in an adventure game of his own, in the apparent hopes that Ray will play the game the player is playing and remember how important Dennis is to him (which is an idea so meta that it makes my head hurt. -- Further proof that Ben was drinking heavily again).
3 1/2) Ray goes off to play Snakes of Avalon while Dennis goes off to do something else. Ray is attacked by the blobs and is later seduced by Square Penis. Ray is now evil.
4) Dennis returns from his ocean adventures to find that Knottybeard the Pirate has taken over Dennis' island in Pirates on the Poopdeck. Ray is mentioned in this game but not seen. (He may have been away working for Square Penis).
5) Dennis takes a trip to the mainland and agrees to undertake a mission for the mayor by defusing a bologna bombwich as seen in Blades of Passion.
6) Shortly after, as seen in The Naked & The Ninja, the Mayor travels to Dennis' island to ask Ray (who is either no longer evil or really, really good at hiding it) to rescue Dennis, who is being held by ninjas in the employ of Hansel*.
7) At some point after that, Dennis and Ray break up and Dennis gets a new Japanese boyfriend. Together they explore a dungeon and defeat the Internet in OSD Gets Textual.
8 ) Millions, possibly billions, of years later, Dennis is rescued by Heavenwhiskers Fred from his magical slumber to begin his RPG adventure. The world has been destroyed and the weak are preyed upon by small dogs and giant demons.
I have no idea where the other games fit in this terribly loose continuity. But I can tell you that my next game, Archipelago Adventures!, takes places after The Naked and The Ninja (and possibly OSD Gets Textual). Dennis finally goes on the adventures he had scheduled back during opening scene of Blades of Passion. Also, Dennis and Ray will be a couple again.
Anyhoo, this just goes to show how wonderfully convoluted the OSD universe is. I think any effort to shoehorn it into a rigid continuity will eventually have the same side effect it had on RON. It will be so hard for any newcomer to know where and when to place their game in the time line that they may not even try.
Plus, as we've seen with Ray, you can do whatever you want with characters in this game and no one has to respect the changes you've made. In RON, it would be terribly rude to kill off someone else's character, so lots of people are running around town protected by script immunity. In OSD, you could have Knottybeard feed Hansel into a woodchipper in one game, and have him return in the next game with no mention of it. That's the beauty of the OSDverse.
Your mileage may vary, of course. But I say you should feel free to whatever you like with OSD. I know I will. Plus, I'm hoping Ben will get really drunk again and make another one. Or maybe Tzachs will fire up the microphone and give us another "Dear Ray..." vocal performance.

- Ponch
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* HANNNNNSSEEELLLLLL!