You're not missing out if you give this one a pass.
You play the role of Barbie, a diver and treasure hunter, who wants to find a collection of items underwater. Some are story-driven, like pieces of a map which lead to the ultimate treasure, while others are simple jewelry which you collect in your personal jewel box, but all of them can only be found by playing a series of mini-games.
Coming out somewhat short of a year after the release of the GBC, Barbie: OD actually looks decent. While it's not a masterpiece by any means, it's one of the few Barbie games on this console to take the time to add quite a bit of detail in the underwater sections you get to explore, with fish moving in several directions, stars spinning, coral reefs and broken ships, just to name some of the details that stand out. Barbie, herself, has more frames in her than all her later games combined. Put simply, it actually looks better than some of the games that came out later.
On the other hand, the audio of this game is a big problem. There are just a handful of basic sound effects in the whole game, and there's not a lot of variety in the music either. There are several themes, one per area and mini-game, but they sound too similar, and some high pitch notes will surely have you looking for the volume button sooner rather than later.
A key example of this happens during the piano mini-game - a "simon says" variant - where the aggravating song plays loud enough that it's distracting from the pattern of notes you're supposed to be listening, which basically means you're better off relying on the visual input alone to complete the mini-game.
The whole game is just a combination of three exploration areas, an open sea, an abandoned ship, and a lost city. Each area has three stages, and each stage has a mini-game. In these exploration stages, you swim around until you find the icon that triggers the mini-game. Completing the mini-game once will give you the story-driven item you need to proceed, but playing it a few more times nets collectibles - which is simply a way to keep score, and you get nothing for your trouble other than a full jewel box.
The nine mini-games are the following:
1 - Drag the Picture Pieces. A simple 4x2 piece puzzle.
2 - Catch the Pearls in the Oyster. Similar to a cross between a pinball screen and breakout, but instead of paddles you get an oyster which you move left and right until you catch a falling pearl. The pearl can bounce off fish or be shot at a random direction if it hits a star.
3 - Help the Baby Turtles. A maze game, where you simply move the baby turtle towards the exit, where their mother awaits.
4 - Touch the Sea Anemone. A group of six anemones stand in a circle. Touching one makes that anemone big or small, as well as some of the others. Touch them until they are all small.
5 - Catch the Cannon Balls. There's a barrel moving left and right and you control when a cannon ball is dropped. Get all the balls inside the barrel.
6 - Fish Twins. Turn two shells to match the fish hiding behind them.
7 - Play Notes. Watch and memorize what notes are played and repeat the pattern.
8 - Catch Bubbles in Rings. Rows of rings move from left to right. Release bubbles so they hit the rings of the same color as the bubble. Get five of the same color in a row to win.
9 - Help Sandy the Dolphin. Same as the Help the Baby Turtles, but now you can't see the whole maze. Set as an exploration zone, move the Dolphin around until you find the exit gate.
Between areas you also get to play a couple of micro-games (I don't think they even qualify as mini-games), the first is picking up an item and putting it in a statue's hand, while the other is picking up three crystals and placing them in three cavities on a door. That's pretty much it.
The game isn't hard to complete at all, and a seasoned gamer will likely finish the adventure in less than one hour, assuming you go for all the treasure. However, if you just want to complete the story, it can be done in 15 minutes or so. Obviously, the target audience will take longer, especially since some of the mini-games provide some challenge that is not commonly found in other Barbie titles.
While I'm sure many children will want to play some of these mini-games more than once, a big flaw in Barbie: OD is that none of the mini-games are playable until you go through the story first. This is an issue that was solved in later games, but kind of sticks out like a sore thumb here.
Overall, Barbie: OD is not a bad game, considering who it's aimed at, but it's not good either. The overly simplistic exploration mode, inability to play the mini-games before going through the story, and severe audio issues, are what make this title hard to recommend except for the more die-hard Barbie fans. Even then, you should probably only consider it if you find it cheap.