I recently discovered a new artist and want to start uploading his art here, but his name ohagi has been used by another artist, so how should I do?
Currently I'm thinking of ohagi_(karakoro_neko) by adding his twitter ID after his name.

iirc situations like this have been sorted out either by switching one artist to possible alternative alias they have, or adding a suffix like _(pixiv) to the other artist's tag (for the record I haven't been here for years, so how these are handled may have changed).

You can either use ohagi_(karakoro_neko) or only karakoro_neko

I would personally choose the source with more material as the main name, for example, if someone's Pixiv is kinda dead or rarely gets new content then I pick the twitter name, but if both get the same content all the time I would do a Pixiv_(twitter) name since usually that one has better quality.

This in the case I can read the pixiv name, otherwise I will default to twitter.

Just make sure that you do something like noting that hurimaro_metayaki is a different ohagi in the artist pages if you have a conflict like this. Someone's bound to try to merge them carelessly.