Transaction becomes irreversible when it is included in a block that is then confirmed by enough different witnesses. Once the node observes enough proper confirmations, it drops related undo session, so there is no way for it to revert to different fork, even if it wanted to.

Before OBI the only way for a witness to confirm a block was to wait for their turn to produce and build a block on top of previous one - that counted as confirmation of all the blocks below new block. For this reason irreversibility only triggered after at least 15 blocks, which is 45 seconds (sometimes more if the chain of blocks contained schedule switch). OBI just created a shortcut - an alternative way of confirming a block (the old way still works). It is done with special transactions that are just not put in the blocks. These transactions still need to be properly signed and only count if the witnesses that sent them are elected for future schedule.