They are rotated/flipped tiles, but I think I cannot understand what OAM is (I am a noob and the only thing I can do is changing some numbers in the code), I'm sorry. Maybe I should learn something about 68K assembly.
In most Nintendo consoles then OAM = Object Area Memory. AKA the area of memory that handles the sprites (sometimes called objects/objs in various programming guides and discussions). The background/BG, which also is what usually is supposed to be handling text, also having something similar.
In its simplest form then it will simply be the X-Y locations of the sprite/tile, however more powerful systems will also give it rotate, flip, scale perhaps, transparency/visibility (though many will just say go off the screen to become invisible), priority, what palette to use, formats and all the other fun possibilities that devs could add to a given piece of sprite bothering hardware. Such details usually then being important for whatever hardware documentation you have or an emulator provides. http://techdocs.exodusemulator.com/Console/SegaMegaDrive/Documentation.html
looks a bit more rough and ready than some of the documents we are otherwise used to for other systems but the main document there looks like it has some good stuff in on layers, sprites, backgrounds, windows and whatnot. Sprite attribute table appears to be the name in those docs but some emulators, hackers or coders might use other things.
Page 55 in the internal numbering, or 65 if you are using the PDF viewer numbering, though before it and after it is good stuff.
You might be able to fix it without knowing much code if you are only getting numbers entered and it is copying those numbers from somewhere else, or is just an instruction saying put this number here (same idea to you probably not knowing how a calculator works underneath it all but still able to enter numbers).