I'm not sure how to get composite video from an Oric. As far as I remember, they output varying signal strength depending on batch and model. I seem to recall even getting a SCART RGB output was a bit of a fiddle with lots of capacitors and stuff just to stabilize the signal. Some of the newer, French made (?) Oric-1's or if those were Atmoses apparently output better signals than the early UK models do but I might might be wrong about this.
The composite video hack as described in the thread might fail if the signal going to the RF modulator is too weak to be used as composite video input to the monitor, it might be enough for the RF modulator to work with. I have read about this for a number of other home computers and video games, that you can't always expect that the RF modulator is fed a nice 0.7 Vpp (or if it should be 1.0 Vpp) signal that you can tap. Even the ZX Spectrum might be on the verge for this hack to work, depending which monitor or TV you use in the end, but usually it works fine.
Which Commodore monitor do you have, an 1702, 1802 or 1902? I suppose in theory there probably are RGB to composite converters, whether they work well with the signal levels output from the computer or not is to find out.