Mars is meh, it's very clearly a corporate colony. Like half the dialogue about it is like "ohz your family has a monopoly on the terraformers" and "financial incentives" and shit. It's like a South Korean military/corporate welfare type state.
The OPA people talk more about liberation using proletarian language "it is who build their ships, it is us who mine their ores and water, and we never get anything in return". Fred Johnson is also pretty clearly an internationalist, he is big on cooperation and leveraging the Belt's labor power to extract concessions and control over the ring gates from Earth and Mars.
The OPA is basically the IRA or the PLO. There are tons of factions within it and some are pretty clearly socialist (Fred) while others are nationalist (Marco) and others are just Mafia style mob bosses (Dawes).