625i for 576i and 525i for 480i is completely normal and expected. The console actually sends 625 lines in PAL mode and 525 lines in NTSC mode, as @paulson explained.
As for the issue with 480p you’re having, it looks like some sort of a sync issue. Does the OSSC itself lose sync or is it actually your television that is acting up? OSSC has a red LED that turns on whenever it loses sync. The LCD also should indicate if the OSSC has lost sync.