When using the Toro, the switch in the bank of four switches that is closest to the console toggles between 15kHz (240p/288p/480i/576i) output and 31kHz (480p) output; so, if the OSSC is reporting that you’re only getting 480i (525i), then you need to flip that switch to 31kHz and restart the console. If you boot a game and says that it’s incompatible with the attached AV cable, flip that switch back to 15kHz, restart the console, and then flip the switch back to 31kHz when you see the white SEGA licensing screen.
What’s confusing is that there’s a second, harder-to-get-to RGBS/RGBHV toggle between the DE-15 and SCART outputs, but that only toggles whether the DE-15 output uses composite sync or separate sync. The SCART output will always output RGBS, regardless of whether the console is outputting 15kHz RGBS or 31kHz RGBHV.
Additionally, I would recommend recommend getting a male-to-male SCART cable and using that to connect the Dreamcast to the OSSC; AV3 lacks low pass filtering for anything below 960p/1080i/1080p, whereas AV1 (and AV2) has low-pass filtering for the video modes used by the Dreamcast, and it’s configured for DTV 480p by default. If you want to continue using AV3, that’s fine, but you should change 480p in sampler to DTV 480p so you get the correct aspect ratio.