There are the following defeat conditions in OpenXcom:
1. Your last base gets destroyed by aliens (or yourself)
2. You fail the final mission
3. You have a negative budget for two months
4. You have a negative score for two months (how much negative it can be depends on difficulty)
All of these are hard-coded and cannot be removed by modding.
You can however create mods to fix it indirectly (instead of removing the conditions, you can change the game so that these conditions will never be true):
1. Mod the damage of Missile Defences to 50000 (original 500), build time to 1 day (original 16), hit ratio to 100% (original 50%) and cost to $1k (original $200k) and add it to your starting base; also when you build a new base, build Missile Defences too
2. Don't do the final mission
3. Mod your starting budget to $100'000'000'000... that should do
4. Mod the score penalty of all alien activity to zero... that way you won't get any negative score
Final words: if you do all this, you just fucked up the game completely... but if it makes you happy, who am I to judge you
EDIT: there are no mods like this... nobody ever needed them... you'll need to write them yourself (I'd say it could take about 30 minutes for point 1, 5 minutes for point 3 and about 1-2 hours for point 4)
