Where the word comes from
Oripa comes from Japanese card shops. sinsa, which runs DOPA! in Japan, describes it as an original pack made by the operator, with several cards randomly included, rather than a pack sold by the card maker.
In English-speaking hobby terms, an oripa is basically a repack.
How online oripa works
You buy points or credits, pick a pack and draw it online. The site shows which card you got and, if you want it, ships the physical card.
DOPA's global site shows the full prize pool and the odds for every card in each pack. That's the first thing to read on any oripa site.
Oripa vs sealed boosters
A sealed booster comes from the card maker with its own print run. An oripa's contents are chosen by the seller, so the seller decides what's in the pool and how many of each card.
That makes the published pool and odds the only real disclosure you get. If a site won't show them, don't buy.
What to check before you draw
Read the full prize pool, not just the top cards. Check whether unwanted cards convert to cash or only to credits, and how long shipping takes.
Compare sold prices for the exact language and grade of the cards you'd want, then set a fixed budget.