OctoMom's first day as a self-serving porn star is in the can ... so to speak. 

TMZ has learned ... Nadya Suleman completed her first day of XXX shooting at a porn mansion in the San Fernando Valley on Thursday ... and we're told things couldn't have gone better!!! 

We're told Octo appeared nervous when she arrived to the home ... but after chatting with some other porn stars who were at the house, she seemed to calm down. 

Before the shoot, producers decided it would be a good idea to screen some porno flicks for Octo as an educational tool ... so to speak. 

But when it was time for Octo to get down to business ... with herself ... one source on the set tells us "She was a natural ... she looked great!!" 

Hopefully Octo has a good babysitter -- 'cause she's due back on the set next week.