OneBlock is a Skyblock variant in which each player owns one block that regenerates as something new every time it is broken. What appears depends on the phase the block is in, so progression is a series of phases, each with its own blocks, mobs and rewards, rather than a list of challenges.
A OneBlock server setup packages the block and its phase system, the island that holds it, and usually an economy, quests and a shop. Because the content comes from phases, the quality of the setup is mostly the quality and balance of those phases.
What to look for in a OneBlock setup
Phases: check how many there are, how long each takes to complete and whether you can edit the drop tables.
Island and world handling: find out how islands are created and stored, since each player has one.
Progression: see what happens after the last phase, whether it loops and what keeps players engaged.
Server software and version: confirm the platform and Minecraft version it supports.
Performance: constant block regeneration and mob spawning add load, so look for recommended player counts.