O2Jam is an example of a game in the VSRG (Vertical Scrolling Rhythm Game) genre. It is a rhythm game where you have to hit the notes falling down (or up), which will appear according to the music, as with every other rhythm game. Think of Guitar Hero or Dance Dance Revolution. Every note corresponds to a music, so as you hit the notes, the actual music notes will also be played!
O2Jam is a 7k game, and mostly focused on Long Notes. A 7k VSRG would mean that there are 7 columns where the notes would fall down.
An example of the playfield is shown on the picture above.
In the playfield, you have the big Combo Meter in the middle, and the judgement just below it. On the right of the playfield are the pills (which helps O2Jam players continue their combo) and the HP gauge. There is the Jam gauge, which is a big influence to the score just below it.
Online O2Jam provides a good multiplayer environment. It is a good place to socialize with other Rhythm Gamers too. You can compete with up to 8 players simultaneously, each games as intense as the song provides.
O2Jam is known to have a very lenient Judgement, which determines how accurate you have hit the note. Compared to other Rhythm Games / VSRG such as LR2 or IIDX, what is defined as a full accuracy play in O2Jam might only translate as 75% accuracy in those games.
Since O2Jam is an old game, expect an old-style interface. You can't play O2Jam on window mode without doing some nasty trick, but there is O2Mania which is an O2Jam emulator.
If you would like to play and compete with others in real-time (and prefer long notes, which are amazing), I would recommend you to play this game.
Here is a high-level gameplay video by Entozer.
Note that I said in the beginning, every note corresponds to a musical note. That is true for official songs.
At higher level gameplay, the notes does not correspond to a musical note most of the time. This is because most of the high level songs are created by other players, who have no time to actually compose the music for O2jam.
If you want to play songs that all have corresponding musical note per key, then there is another VSRG called Lunatic Rave 2, which is a Beatmania Simulator (BMS) for the PC.
Alright! I'm done with the necessary explanation, and that's good and all, but how do you play this? Where can you get this? How do you start playing?
Since the official servers are down, you won't be able to play it unless you go for private servers, a server emulator (o2Emu), or o2mania.
If you want to know more, check out these guides!
O2Jam Gameplay Guide: http://ecilasworld.blogspot.com/2014/07/o2jam-gameplay-guide.html
O2Jam Skill Rings: http://ecilasworld.blogspot.com/2014/06/o2jam-skill-rings.html
O2Mania Guide: http://ecilasworld.blogspot.com/2014/06/o2mania-guide.html
O2Private Servers: -Coming Soon!-
Feel free to comment or ask any questions!