My hope was that OSSC would have this feature.
*My XRGB2 does this as well.
The OSSC and XRGBs are different machines for different applications. While both are intended to digitize analogue AV signals, the XRGB machines are full scalers, in that they can scale and shape the image to your preference, at the cost of video lag; whereas the OSSC is a line doubler, which can enlarge the image and tweak certain characteristics with imperceptible lag.
Ultimately, you will need either an external converter (like this) or track down a TV that supports 50Hz video. (A bit of a pain in North America, but not impossible.)