The standard spread (252 SpA, 20 Atk, 236 Spe Naive nature) already garantees the OHKO on Mega Ttar after rocks:We put 136 in Atk for a guaranteed OHKO with Stealth Rocks down on Mega TTar
20 Atk Life Orb Deoxys-S Superpower vs. 4 HP / 0 Def Mega Tyranitar: 312-369 (91.2 - 107.8%) -- guaranteed OHKO after Stealth Rock
OHKOs standard defensive Mega Venusaur after rocks (81% chance without rocks):
252 SpA Life Orb Deoxys-S Psycho Boost vs. 252 HP / 188 SpD Mega Venusaur: 351-416 (96.4 - 114.2%) -- guaranteed OHKO after Stealth Rock
And even max Calm SpD Mega Venusaur will usually be OHKO'd after rocks:
252 SpA Life Orb Deoxys-S Psycho Boost vs. 252 HP / 252+ SpD Mega Venusaur: 307-361 (84.3 - 99.1%) -- 75% chance to OHKO after Stealth Rock
Honestly there's very little reason to run your spread as it doesn't seem to get any extra OHKOs/2HKOs (even missing out on some) and it sacrifices the speed it needs to reliably revenge kill things like Scarf Garchomp.
Edit: Knock Off is becoming increasingly popular on Deoxys-S as there's less need for HP Fire/Fire Punch than last gen due to the steel nerf and Knock Off is easily the best attacking move in this gen.