The Gacha in OPTS is pretty straightforward. It begins with listing the coin Gacha(s). A single pull costs 5 rainbow coins. Using 50 coins often gets you 11 cards, but the bonus may vary. The date and time the Gacha will end is also written, so make sure that you don’t miss out on the Gacha you want. The coin Gacha often has a base guarantee of having a pool of only 3-star or higher rarity cards.
You can buy coins with real money in the Shop, and there are shortcuts to edit your party with your new cards or return to the Plaza you were in. The gacha regulations are there as part of Japan laws on IAP Gacha.
ETA: A Gacha readme has appeared on the Gacha page to state that if you have not gotten a 5-star card from the coin Gacha after “a certain amount of pulls”, the “original rates” of 5-star will increase slightly and those of 3- and 4-star will decrease proportionately. Once you receive a 5-star, the “original rates” will reset to normal.
What is important is the List of cards that shows you all the cards in the current gacha and the respective rates. You can press and hold each icon to see the details of the card. There is also a disclaimer that, among the same rarity tier, the rate can actually be different (i.e. card A may be more common than card B even if both are 5-star).
Friend-Point (FP) Gacha
You gain 20 FP each time you play co-op with another player (30 FP if they are your Friend or Ally), capped at 500 a day. Unlike the coin gacha, you do not get to see the pool or enjoy bonus if you pull 10 at a time. Players have reported getting anything from 1-star to 5-star regular scene card (albeit very, very rare). Each pull costs 50 FP and if you have 500 FP or more, you can pull 10 at a go, or up to the amount of FP you have.
Understanding what you got
This is for beginners who sometimes get confused. On your Gacha result screen, if you see a small icon on the bottom right of the card, this card has a skill or Special of someone attached to it. Otherwise, it’s just a regular scene card and its main purpose is to give stats boost to your character. For more details, see Scene Cards.
A 4-star Special card for Luffy and a regular 5-star card