The output resolution is determined by the game, most PSX games will always run at ~240p aside from the occasional high resolution menu. However, the PSX is capable of 480i output and some games use this for gameplay(mostly later racing titles). The OSSC just takes what it is given, so there’s nothing you can really do about it other than optimizing your settings and timings for PSX 480i, using the vertical mask to cover the flickering top and bottom lines. If it’s still bothersome, your only option is to use a display with good deinterlacing and set the OSSC to passthru for 480i. I use a mid 2000’s Sharp Aquos for this as it has an excellent deinterlacer that shows no flickering, with a latency cost of just 12ms.
I use a well made PS3 component cable for my PS2 and the picture is razor sharp when tuned. I’ve heard some of the cheaper made PS3 cables don’t support PS2, you may consider buying a better cable. Don’t be afraid to spend $50+ on an OEM cable, as cheaper cables are unshielded, resulting in colors blurring together into a darkened mess(dark levels often become unplayable). The RGB SCART cables don’t support 480p+ on PS2, but I’ve never used them so I don’t know if that’s a problem here.
I wish JunkerHQ would come back up because I need the timing sheet to properly set up one of the many PSX resolutions I don’t have a profile for.
EDIT: I just remembered Castlemania Games sells Retrovision HD cables for PS2/3 at $30, so those would be an excellent non-OEM choice.