To write a good 'callen oc fanfic', it's important to have a solid plot. Maybe start with a conflict that involves both Callen and the oc. For instance, they could be on the hunt for a dangerous criminal, but the oc has some personal connection to the criminal that complicates things. Also, make the dialogue between them realistic. They should have different voices and ways of expressing themselves.
First, know the character of Callen well. Study his traits, his backstory, and how he behaves in different situations. Then, create your oc with a clear personality. For example, if your oc is shy, show how that affects their interactions with Callen.