@Fuck This dude! As the dude you're leading, if the situation feels awkward it's because you let it. I know her behaviour plays into this, it's holding and maintaining your frame. If she's awkward and skittish that's now on her.
Being social, warm and welcoming counteracts this. If you're not ashamed, embarrassed, nervous, awkward about seeing/meeting her and you view it more you're meeting/seeing a girl you know give her a warm inviting smile and being cool about it she'll feel it.
If she acts weird and looks away or does anything squirmish just hold your frame. She's cute and silly, you're cool and collected. It happens, like running into your ex's. The more confident you are with this (because you've experienced it 1000 times) the more at ease she'll be.
Your eye contact and body language will say it all, but you can also give a warm hi. Act as if she's a girl you met in line at the club, not one you fucked.
With the emphasis on a ONS I'm assuming no contact details were exchanged to give an after sex text. The after sex text is your best bet otherwise dude!
Edit* after sex text can be found in the post linked by
@Dr.X and is covered in the text article 14 texts to send a girl.
The awkwardness is her ASD and trying to backward rationalise what happened. Overcome this by being the same sexy dude and being unfazed.
If you've text her and she hasn't replied handle this in the same way.