For me, only one was literally only one, I used the number 1 for entire game, points and power-ups. When u take a power-up, fow example your speed goes from 1 to 11 immediately. But same thing happens to your score too. So you need decide carefully while taking a power-up. They are a little much rewarding :D Here is the game, you can play and rate it!

https://itch.io/jam/gmtk-2019/rate/462865

My "One" is a flag, a blue flag.  You play as a little blue guy from a very blue land. It has been taken over by evil red guys, who have committed the horrible crime of wearing a different coloured shirt from you. What's worse is they have planted their repugnant red flags all across your beautiful blue land! This cannot stand. You will not rest until every red flag has been captured, and all the land secured under one flag, the one true flag, the blue flag!

https://itch.io/jam/gmtk-2019/rate/462780

My "Only One" was "Only One Resource": Hearts. They're your life, ammo, and timer. I also wanted to add dashing with the hearts being stamina, but I barely made the game as is in this timeframe.

Click HERE to play the game: HeartGame

I game that you can ONLY play ONCE (it is actually a meta-puzzle-game)

https://itch.io/jam/gmtk-2019/rate/461276

Only one developer, only one day (sigh), only one dimension (at a time)

https://doctormikereddy.itch.io/thomas-was-a-line and https://itch.io/jam/gmtk-2019/rate/462856

Can only move in 1 direction, entire game including menu's and level built into only 1 Unreal level.

Our game is build around "Only one damage" or "Only one strength" concept.

We came up with a pretty cool damage effect. It's a lot of fun. And I still stuck at level 18.

https://santazaaa.itch.io/hyper-slash

Originally I was planning to make a mixology game where there was only one drink you could make that wouldn't cause your patron's head to fall off, but the files all got corrupted and I didn't have time to finish -_- - so I made a crafting game, Decrafted (https://itch.io/jam/gmtk-2019/rate/463986) where the objective is to craft your way down to one item rather than up to as many as possible

For my game If (https://itch.io/jam/gmtk-2019/rate/461616), an Gameboy RPG game, started with: one action button, one character and one place.

In the end finish with one action button, one character, one place, one enemy, one NPC and one battle.

We made a two-player game called Cyclopia featuring shooting  with only one eye. Literally with one single eye you could not perceive the depth. But you can estimate the distance of the bullet from other hints to bounce off back your opponent like Genji.

I did one huge map xD https://itch.io/jam/gmtk-2019/rate/460859

I did  a text adventure titled "Only One Day", where you live your final day before the end of the world! Jolly, right?

https://brownseededbread.itch.io/onlyoneday

For my solo roguelike project, the only one thing was only one item in inventory :D

Our two person team dug up a rough concept we'd been kicking around but never done anything with. "You had ONE job" is a multiplayer, arena/horde based game where you can only do a singular thing. Shooting, or looting (we wanted to add "defending/healing" but ran out of time). The idea is that you can't accomplish anything on your own. If I keep working on this I want to improve individual player scoring and maybe to end game callouts (Like Overwatch in its voting) and implement the missing class, and generally improve the feeling of you doing a singular thing and playing a role in a larger team.

https://itch.io/jam/gmtk-2019/rate/461546#post-854292

Here it is if anyone wants to take a look, be warned that controllers and multiple people are kinda required to get any kind of experience out of it. Probably not the best for a game jam game but we're endlessly inspired by couch co-ops like Overcooked and Ultimate Chicken horse.

"LASER" As the minimalistic title of my game suggests.

https://guillemsrr.itch.io/lsr

I wanted to pick an idea that would be seemingly impossible with "only one." I ended up doing a text adventure with only one word, and I think it turned out pretty well! https://itch.io/jam/gmtk-2019/rate/461221

In Whiteboard Arena my "only one(s)" were: one vs one with one keyboard, only one action (jump), only one screen, last one standing.

https://itch.io/jam/gmtk-2019/rate/461347

For our game, a maze with only one exit: https://itch.io/jam/gmtk-2019/rate/463810

Sp00ky game where you have only one second to see at a time (lightning strikes)!

https://itch.io/jam/gmtk-2019/rate/463099

For me it was make a platformer with 'Only One' thing to climb. I wanted it to be a creature, but ended having to settle for on massive and complicated wall instead, i learned a great deal about design in the process!

https://newkryptonite.itch.io/the-lonly

After writing 3 pages of ideas and sketches I picked the funnier one that would create more mechanic based on something very simple: Only one eye, leading to "half the eyes, half the screen" mechanic 

https://itch.io/jam/gmtk-2019/rate/460908

I couldn't decide about my only one. So, in my project I used movement in only one direction and only one button for control. Also I tried to use only one color, but actually there is at least two of them: background and foreground. So my game is black and white.

In Ninja On A Budget , you only have a single shuriken to defeat all the enemies!

So you must retrieve it from the corpses after each use... Check it out!

My project was about the last One in a world full of evil Zeroes, and had only one core mechanic, turning on some power switches! These are the little guys

We'd love to have you guys give it a try!

https://itch.io/jam/gmtk-2019/rate/463638

My game is a platformer that uses only one button to control everything.

Mine (https://h-labs.itch.io/only-you) was you being the only thing in the level (aside from the goal). Once I read the theme, I thought "how could you complete a level if you're the only thing in it?" and ran with it.

Through a fun twist of fate, I also ended up with one-button controls.

For my game, "Only one" is the bullet, the minutes and the enemy. 

In the game there is several pnj but only one is the enemy you have to find him in 1 minutes and shoot him with one bullet 

https://itch.io/jam/gmtk-2019/rate/460819

When I heard 'Only one' I immediately went to 'THERE CAN ONLY BE ONE' and the elimination aspect of it, going from having a lot being whittled down until there is only one(or almost only one anyway). Thinking of it like a single player battle royal or a musical chairs like game where you start up controlling a load of characters, until the environment reduces it down to potentially only one as you sacrifice multiple character to solve puzzles to save that one.

From that I swapped musical chairs with zombies that would slowly assimilate your crowd, and while I deiced not to be a stickler of forcing the player to have to reduce there crowd to one person to pass the level, they only needed one person to survive to win. Apart from that, I just tried to keep everything minimalist, colors are mostly all green, all the charters act as one and the like.

My game a quick to play zombie survival puzzle game; https://itch.io/jam/gmtk-2019/rate/461074

For my submission I tried to get as many mechanics into one button as I could.

I made a 2D skateboarding game and I was able to fit jumps, grinds, flip tricks, grinds tricks  and manuals into my only button.

My game's original "Only One" was to create a set of puzzles that required only one key press to solve. And that remained true in the final product.  What did evolve though was how much I wanted to push that central gimmick. I came up with all kinds of weird puzzles by the end that still stay true to the gimmick but also (hopefully) changed the player's perception of it. 

Unfortunately, some of the early puzzles I designed were too obtuse for players who can't read my mind (whoops!). The latter puzzles seemed more intuitive in testing, but my biggest regret of the jam was not revising those introductory levels (the answer to the ladder puzzle is "M" for anyone who wants to check the game out).

https://itch.io/jam/gmtk-2019/rate/461968

My entry's "only one" is the player's only one ability, which is to jump (more than once, though). And, also, the game's playable with only one button.

I intended to have more stages and a recurring boss that you would have to fight in different ways each time (and an one-legged character for extra "one-ness"), but the deadline lazyness struck me hard. Still, I'm glad I managed to submit something in the only one (HA!) Game Jam that I participated.

One hand gameplay, with one input. And.. there is 1 hand model  in game. It is just a simple platformer that allow to play like traditional arcade shooter moving at fast pace, zipping through levels. I learnt a lot of new thing from this gamejam within short period of time.