https://mods.factorio.com/mod/omni

This mod and Omni are incompatible.

This is a very unhelpful bug report, in what way are the two mods incompatible? Are you receiving an error on load, while loading a save, or while playing the save itself?

Game loads up fine for me with just OMNI and Redrawn Space Compatibility:

The error I get is this:
Failed to load mods: Error while running setup for space-location prototype "OMNI" (planet): medium-rock (decorative) does not have an autoplace specification.

Disabling Omni fixes this issue.

That's odd, do you have any other mods running besides just RS Compat (and its requirements) and Omni? Are you on the latest non-experimental version (v2.0.72)? Are you able to load up the game with just Omni by itself?

Have you tried verifying files (for the Steam version), or redownloading the game from the Factorio website (for the standalone version)? It's possible you have missing or corrupted files that are affecting things, because the game loads for me with RS Compat v1.0.28, RSC v2.3.2, PlanetsLib 1.14.7, PlanetsLib: Tiers 1.2.0, and Kryzeth Standard Library v2.0.11 (latest version of all required mods)

Looking back at the OMNI discussion threads, your issue appears to be a duplicate of this one, which had already been reported. The real issue is OMNI being incompatible with Alien Biomes, nothing to do with RS Compat

Alien Biomes is not recommended for Space Age, as several planet mods are incompatible due to their removal of basic decoratives like the one mentioned in the error message. It's not just OMNI