The quick answer is that the OSSC is not the correct tool for this job. I recommend picking up a cheap composite/s-video to HDMI converter on Amazon or eBay, as that will accomplish what you want. Just make sure it supports PAL.
The longer answer:
…Composite Video connector to a SCART adapter -> OSSC…
This is the problem–you’re feeding composite video into the OSSC, which was designed to only accept RGB and YPbPr. Those composite video to SCART adapters are simple, passive gender changers. Neither these adapters nor the OSSC have the capability to decode composite video (nor S-Video nor RF, for that matter) to something usable by the OSSC.
The reason the OSSC says it has sync, but you’re not getting any video, is because it can and is using the composite video as a sync signal. The SCART adapter is putting composite video on the sync line and nothing on the R, G, or B lines, so the OSSC gets a valid sync signal but ultimately has no video data to display.
If you really want to use the OSSC for this, you will need to track down an RGB decoder (So, something like the Kramer FC-4044 or DVDO iScan Plus; which are really expensive and hard to find now) and wire it between your video recorder and the OSSC.