So, we're playing as as small community and have our modpack with OARC scenario (https://mods.factorio.com/mod/oarc-mod) and Diplomacy mod (https://mods.factorio.com/mod/diplomacy). I think something is messing with relationships of factions and now all biters are neutral to players. Is there any kind of command or script that i cat add to save that will make all biters hostile toward players?

you can use the forces and command line scripts to set relationships to 'enemy' to other forces. ask for help on Factorio discord, basically the players are probably in the "player" faction (i don't know what your mods are doing, sorry) but if they are in other forces that's fine too, you just need to know their name.

you can then set the state of each force relative to each other force, to enemy.

Note the biter factions are forces called: "biter_faction_x" where x is 1,2,3,4,5 for the default of 5 biter factions.

there's a LuaForce api you can use in the script interface, to check if a force is set to friendly or neutral/cease-fire with another force. You can 'get' the current status and print it, to check what it is now, and then use the 'set' to change it. https://lua-api.factorio.com/latest/LuaForce.html