Thank you for replies.
It is shame that I didn’t research this problem before I purchased an OSSC. I hope the OSSC will include a quality mode for interlaced resolution. (It may need an additional hardware chip.)
I found that a ‘GS mode selector’ can add a native 240p/480p output resolution for the 480i PS2 games. It might be a solution for my problem.
Thank you again guys.