Can i run OpenHAB in parallel with other websites which control an ESP8266 separately but on the same Raspbery Pi? What's the best way to do this?

Setup, Configuration and Use Beginners
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You can use lighttpd, nginx or even apache2 in parallel to openHAB as long as you take care of different ports (easy enough, as openHAB uses 8080 and 8443 per default). I would suggest not to use apache2 as its footprint is measurably bigger than lighttpd.

Did you see this topic? There are even more (though mostly openHAB specific) here