o2em <file> [options]
The file name must include the extension, which is .bin  
The most commonly used option would be -fullscreen
Usage:
o2em <file> [options]
<file> = file to load with extension
options:
-wsize=n      Window size used by the emulator (1=original, 2=double size, etc.)
-fullscreen   Use full screen mode
-scanlines    Enable scanlines use by the video emulation. Scanlines are a
              feature that makes your display look like a TV screen.
-nosound      Turn off sound emulation
-novoice      Turn off voice emulation
-svolume=n    Set sound volume (0-100)
-vvolume=n    Set voice volume (0-100)
-debug        Start the emulator in debug mode
-speed=n      Relative speed (100 = original speed)
-nolimit      Turn off speed limiter
-bios=file    Set the file name or directory to find the console bios.
              By default it looks for a file named o2rom.bin.
              You can use a bios file from an Odyssey2, Videopac or
              Videopac+ console. If you want to run VP+ games (in VP+ mode)
              then you need a VP+ bios.                            
-euro         This option enables the use of European timing / 50Hz mode.
              This option is usually not needed as most of the games that
              really require this mode will be auto-detected.
-filter       Enable the low-pass audio filter.
-scshot=file  Set the screen shot file name or template. The screen shot
              will be saved when you press the F8 key in the emulator.
              The extension of the file you give will set the file type.
              supported file types are bmp, pcx and tga. You can also give
              a template to save several files, using the @ character.
              Using an option like -scshot=dump@.bmp will save files with
              names like dump00.bmp, dump01.bmp, etc. The pictures will
              have a resolution of (320x240)*wsize.
-exrom        Enable the use of an alternative ROM mapping for 4Kb games, to
              support some games that use a special 3kb program rom/1kb data
              ROM mode. The only games I know that use it are Four in 1 Row
              and Musician and both are detected by their CRC and set
              correctly (I least the versions I have). So this option is to be
              used only with games that are currently unknown to O2EM. Do not
              enable it as default, as it will make all the 4kb games that do
              not use this special mode crash.
              
-s1=mode/keys Define stick 1 mode/keys
-s2=mode/keys Define stick 2 mode/keys
              These options define how the console joysticks are emulated.
              You can use a joystick connected to your computer or use
              the keyboard. You can specify a mode number (compatible with
              previous versions of O2EM) to disable the joystick emulation,
              to select an actual joystick or to set a default keyboard
              emulation mode :
                0=Disable,
                1=Default Right keys (arrows keys and right shift)
                2=Default Left keys (W,S,A,D,SPACE)
                3=Joystick
              Example: -s1=1 -s2=3
                                 
              You can also specify a list of 5 keyboard codes that will
              be used to emulate the joystick, separated by comas
              (without spaces), using this order : UP,DOWN,LEFT,RIGHT,FIRE.
              The following codes are accepted (not case sensitive):
                A,B,C,D,E,F,G,H,I,J,K,L,M,N,O,P,Q,R,S,T,U,V,W,X,Y,Z,
                0, 1, 2, 3, 4, 5, 6, 7, 8, 9,
                0_PAD,1_PAD,2_PAD,3_PAD,4_PAD,5_PAD,6_PAD,7_PAD,8_PAD,9_PAD,
                TILDE, MINUS, EQUALS, BACKSPACE, TAB, OPENBRACE, CLOSEBRACE,
                ENTER, COLON, QUOTE, BACKSLASH, BACKSLASH2, COMMA, STOP,
                SLASH, SPACE, INSERT, DEL, HOME, END, PGUP, PGDN, LEFT,
                RIGHT, UP, DOWN, SLASH_PAD, ASTERISK, MINUS_PAD, PLUS_PAD,
                DEL_PAD, ENTER_PAD, PRTSCR, PAUSE, ABNT_C1, YEN, KANA, AT,
                CIRCUMFLEX, COLON2, KANJI, LSHIFT, RSHIFT, LCONTROL,
                RCONTROL, ALT, ALTGR, LWIN, RWIN, MENU, SCRLOCK, NUMLOCK
              Example: -s1=y,h,g,j,lcontrol -s2=8_PAD,5_PAD,4_PAD,6_PAD,RCONTROL