Answer 1

You mean increase the "Accuracy"? Then yes obviously. Though first, their accuracy is increased by 1% for every level higher you are too them starting at 30% Source.

Some good ways to increase Acc for OHKO moves are:

1. Pair in a double battle with a Pokemon with No-guard (Machamp/DW Golurk)
2. Acc boosting moves (Hone-claws, and Coil)
3. Evasion lowering moves (Defog)

Though be warned most of the time OHKO moves are banned from competitive play.

Answer 2

the accuracy of all OHKO moves is decided by :

users level - targets level + 30

so higher your level more the accuracy

Comments

I can't believe no one put this, but don't forget about ***Lock-On*** & ***Mind Reader***, unless you looking for ways to increases the accuracy for the whole battle, not just one turn.

> The next move made by the user of this move will not miss even if the target has used a move similar to Fly. However, if the target uses a move such as Detect and Protect, the user's next move will miss. Also if the target switches out, the effect of Lock-On/ Mind Reader wears off.

Sources:
[Bulbapedia/ Lock-On][1]
[Bulbapedia/ Mind reader][2]


  [1]: http://bulbapedia.bulbagarden.net/wiki/Lock-On_%28move%29
  [2]: http://bulbapedia.bulbagarden.net/wiki/Mind_reader

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You did not add anything on to my answer buy Lock-on and minder reader.

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so ur saying this should have been a comment??

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Yes, yes it should. I just thought you might want to know why  before I turned it into one.

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lolz thank you for being so nice first :)

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I just realized how much Bulbapedia helps you get points UberPwnage. I wish I was as good with resources as you.

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There are people much better than me on this site. Besides, if it's not on Bulbapedia, i'm lost!!!!   XD