With no modifications, I believe the 2600 only outputs via RF, in which case you need to run it through a VCR to convert it to composite, and then you need something like an RT2X plus an HDMI->VGA converter, or a VGP Koryuu (whenever they come out), or something else that can get you from composite video to either YPbPr or RGB.
With a composite video mod, you again need an RT2X plus an HDMI->VGA converter, VGP Koryuu, or something else along those lines.
Lastly, if you have skill and/or money, Tim Worthington has an RGB mod for the 2600; however, it requires a similar installation to the NESRGB (desolder processor from motherboard, socket processor to RGB board, socket RGB board to motherboard), so it’s not for the faint of heart (or wallet).
@BuckoA51 – I’m curious, what settings do you use with your 2600RGB and OSSC? I’ve installed the RGB mod (using the component out) and it works OK w/ my OSSC, but I have an issue where some games (such as Entombed) where the TV just can’t seem to keep a picture going. I assume it’s b/c the game is playing kind of loose with the NTSC spec and after line doubling the signal isn’t stable enough for the TV to stay locked on. But I’ve also not messed with the OSSC’s settings much. All I’ve done is set H=PLL pre and post coast to 3, per http://junkerhq.net/xrgb/index.php?title=2600