But I believe that even Pyshock and Psystrike don't OHKO Blissey.
commented 3 minutes ago by Crimson1
252+ SpA Keldeo Secret Sword vs. 252 HP / 4 Def Blissey: 1256-1478 (175.9 - 207%) -- guaranteed OHKO
Keldeo OHKOs. However do note, Blissey with 252 Defense And a postive Defense Nature will survive it.
252 SpA Mewtwo Psystrike vs. 252 HP / 4 Def Blissey: 765-901 (107.1 - 126.1%) -- guaranteed OHKO
Same case as Keldeo's Secret Sword
The closest you will get WITHOUT ANY DROPS OR INCREASES IN SPECIAL ATTACK OR DEFENSE is probably this;
252+ SpA Choice Specs Kyogre Water Spout (150 BP) vs. 252 HP / 252+ SpD Blissey in Rain on a critical hit: 589-694 (82.4 - 97.1%) -- 62.5% chance to OHKO after Stealth Rock
Yes Specs Kyogre in Rain is stronger than Specs Kyurem-W
I know you said that Psyshock isn't counted but Psyshock is lower base power than Psystrike, and can OHKO too;
252+ SpA Mega Alakazam Psyshock vs. 252 HP / 4 Def Blissey: 742-874 (103.9 - 122.4%) -- guaranteed OHKO
252 SpA Choice Specs Alakazam Psyshock vs. 252 HP / 4 Def Blissey: 832-979 (116.5 - 137.1%) -- guaranteed OHKO
Anyway to sum up - No, nothing can OHKO Blissey as an unboosted Special Attack barr Secret Sword, Psystrike, Psyshock and Water Spout by Specs Kyogre on a crit in rain with Stealth Rock support.
However do remember, once you factor in boosts and drops, many special attacks can kill Blissey. For example;
+6 252 SpA Gengar Sludge Bomb vs. -6 252 HP / 252+ SpD Blissey: 1371-1614 (192 - 226%) -- guaranteed OHKO