On the Toro, you need to have the Dreamcast mode switch (the one in the bank of four closest to the console) set to ‘VGA’, so the console will output 31kHz RGBHV (480p). Then you need to run a fully-wired male-to-male SCART cable from the Toro to AV1, and change the input on the OSSC to AV1-RGBS. The other RGBHV/RGBS toggle, the one between the DE-15 and SCART outputs on the Toro, only changes between RGBHV and RGBS on the DE-15 output; the SCART output is full-time RGBS, regardless of what the console is outputting.
Thanks for the clarification. I will double check when I get home later, but I recall getting a 480i signal regardless of the switch being set to “RBG” or “VGA”.