Here is the exact settings to best re-create that original Vanilla DOS X-Com: UFO Defense experience:
Play Intro at Startup: YES
Explosion Height: 0
Wounded Defend if above %hp: 100%
And everything else in the advanced options would be set to no. To truly replicate the DOS game experience, you would ideally limit to 30 FPS I believe, and also there would be no mousewheel scroll/usage or night vision at all.
Mods: Limit Craft Item Capacities only
Under GEOSCAPE > Globe Details, turn off Radars
Under BATTLESCAPE > Path Preview, turn off both options
Under BATTLESCAPE > UI Options, turn off Tooltips and turn on Death Notifications
Under AUDIO, turn all three volume sliders to max.
Under AUDIO > SFX Format, set to 1.0
Hypothetically, setting language to English (UK) should give you the old original text, but actually you will not only have all of the typos fixed, but much of the British spelling will be replaced with American spelling...last I checked, anyway.
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I think that should just about do it.
This will vary from the original DOS game in several ways:
- it is much less buggy (still buggy, but mostly balance problems, not glitches)
- no more stack overflow
- less crashes, should be no hangs at all
- clock-based timing instead of updates-based timing
- much higher graphical fidelity, either crisp pixels or screen filters of your choice
- Research tree in TFTD is a best-of-all-versions iteration
And the best change of all:
- If you don't like any of these "features", you can change the setting or install a mod!
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I'm looking for experience just as UFO1 regarding the difficulty of game.
Which settings are the same as UFO1? (I do not want to use "Restore Defaults" since I might want to keep customization, and also does this option mean identical to UFO1?).
Especially:
* storage limit for recovered items - original was YES - it was limited and lost, right?
* live alien sale - I think it was allowed ofc, right? so, YES
* Sneaky AI???
* allow psi-capture was OFF?
* allow psi-strength improvement was.... ON?
* override line of fire was OFF?
In addition I think there were one more hurdle in UFO1 which can not be turned on - item limit was 80 right?
and related second hurdle - all items needed to fit visually in the inventory grid on layout or you in some cases couldn't grab them...?
Tho tbh I am not looking forward to adding/using myself these 2 options 
Guess will play with override-LOF option and bump difficulty a from lowest to the 3rd.
The original game was very wonky. Storage limits only affected attempting to purchase something, or remove something from a craft. Items coming back from missions would happily go right past the limit. Alien containment limit did nothing at all. I'm not even sure they remembered to put the value into the game. But the UFOPaedia
said it only holds 20 aliens.
There was no live alien sale, or killing, or even a UI for dealing with them. If you finished researching that specific alien to the point that it had nothing left to give, I think it just stopped showing up that you owned one, and you couldn't get rid of it, not that it mattered. Though it probably would have made you unable to remove your alien containment.
Sneaky AI, like the vast majority of the options, was made by fans later on, not by the original developers. Same with any auto-end-turn effects. Back in the old days, we would spend ten minutes finishing our turn only to find out all of the aliens were already dead or unconscious. There was no psi capture.
There was no way at all to boost psi strength. Only thing you could do was get a new soldier and have them sit in psi training for a month. But because it procs AT month changeover, if you don't get enough good ones, once you sack the spongebrains and get a new batch, you have to wait until the END of the month to START training the next batch. Very slow.
Override line of fire did not exist back then. If the soldier didn't have a clear shot, they could not take the shot. But we worked around it by figuring out what the obstacle was and shooting it directly.
The craft item limit was 80, yes. You can put it into OpenXcom. My own mod uses craft item limits (though much higher). There's a built-in mod just for this.
and related second hurdle - all items needed to fit visually in the inventory grid on layout or you in some cases couldn't grab them...?
There were inventory bugs that aren't in OpenXcom. Aliens with multiple ammo clips (such as blaster bombs) would have them bunched up into the same slot, specifically the left leg pouch. If you picked up one of these and were unable to place it anywhere, you had to crash the game (ctrl+break) and reload from your last save. You could also have just enough items on the ground to fill one page without starting a second page. When this happens, hitting the scroll button does not open up a free page and you won't have room to put stuff on the ground. This is a problem even in openxcom, but they both have different workarounds. In the old game, if you click the scroll button while holding an item in the cursor, it would place that item on the next page, thus creating the page and allowing you to navigate to it. In OpenXcom, you can ctrl+click an item in inventory to drop it on the ground. And in both games, you could occasionally cheese the inventory tetris algorithm by fitting stuff all on one ground page that the algorithm will need an extra page for, then just hit the scroll arrow to update it.
And the old game would display the alien inventory sprite as the first X-Com sprite (blonde male, jumpsuit, AKA "Guile"), whereas OpenXcom displays it blank (because this enables you to add inventory sprites for aliens with mods).
