What do you mean by "OSR"? Not just asking what the acronym means, but what does OSR mean to you? E.g. only D&D versions older than 20 years? What about Traveller (SciFi, not D&D compatible)?
By mindset I assume stuff like "don't be too attached to your character because death is likely" and "GM is impartial referee and must never fudge a roll".