One subtle detail in the way Ethereum works is that there is a difference between Ommers and Non-Canonical Blocks. It’s common for people to use the term Ommer for both of these and most of the time the difference doesn’t matter but sometimes it does.

So what is a non-canonical block? Non-canonical blocks are ones which a client imports but which don’t wind up on the canonical chain. Maybe they were on the canonical chain for a while and then a re-org switched to a different chain or maybe they were imported to a fork and spent their entire life languishing there. Either way, they don’t form part of the current consensus chain and have absolutely no effect on the world state. It’s like their transactions have never been executed and no one gets any form of miner reward for them. Non-canonical blocks must be entirely valid blocks that could form part of the canonical chain, but weren’t because we found a better chain.

Apart from not getting any mining rewards, that probably sounds a lot like an Ommer, and it is. But actually an Ommer is just a block header which has been included in the Ommer list of another block. The block does not have to be completely valid – it just has to have a valid proof of work solution and be from a fork that started within the last 6 blocks of the block that included it. The transactions are not included so there’s no verification that they were in any way valid. The miner of the Ommer gets a small reward when it is included as an Ommer and the miner of the block that includes it also gets an award for doing so.

A block can be both an Ommer and a non-canonical block, and in practice usually are. However, it is possible for a block that would be invalid to import as a canonical or non-canonical block to be included as an Ommer.