Hm. OOC pairings... This is a hard one. Because I'm often a rare-pair 'shipper & reviewers have often told me that I've made some very unlikely pairings (e.g. Grawp/Dennis Creevey, Tonks/Olympe, Lucius/Moody, Remus/Fridwulfa etc) believable... one of the best compliments a writer can be paid! :D

But the pairings I think are extremely OOC are:

Lucius/anybody Muggle-born. Because he's probably too snooty to want to touch a "Mudblood".

Lucius/any Muggle. See above.

Snape/Tonks. Not just 'cos it's het & I'm not really a het fan, or because I dislike the pairing, but I just can't see it happening.

Any giant/any house-elf. Do the math. ;)

There are probably more, but those are the main ones that come to my mind.

Some believable pairings for me: Lucius/Draco (the ultimate in pure-bloodedness!)
Harry/Hagrid. A lot of emotional closeness there...
Moody/Shacklebolt. Auror bonding...
Lucius/Macnair or Lucius/Snape - Death Eater bonding or Slytherin bonding
Ginny/Tonks - sweet girly closeness. Awww...
Lucius/Arthur. The ultimate love-hate relationship...
Dobby/Winky. Cute despite all the dysfunction.
Filch/Mrs Norris. Hee. Just hee.
Cho/Luna. The Ravenclaw-ness of it all...

Love & Serpents' Kisses,
Anath.