Yes you can.
The default OpenGL viewport is (-1,-1) to (1,1) though - when you use Graphics instead of bglCreateContext it sets it to (0,0) to (screen_width,screen_height). So if you're drawing a triangle that would fill the screen with the standard OpenGL viewport, it might just be displayed as one pixel in the top-left corner of the screen - or not at all.
The default OpenGL viewport is (-1,-1) to (1,1) though - when you use Graphics instead of bglCreateContext it sets it to (0,0) to (screen_width,screen_height). So if you're drawing a triangle that would fill the screen with the standard OpenGL viewport, it might just be displayed as one pixel in the top-left corner of the screen - or not at all.