How does the idea of being "star crossed" relate to both romeo and juliet and oedipus the king"

Well star crossed refers to lovers who can't be together and that their end was tragic, yet somewhat romantic. Judging from the fact that both Romeo and Juliet die because of their old family fued, and when Jocasta realizes that she is Oedipus's mother and that Laius was his father she is so shocked witth the outcome of eveything that she takes her own life.  Then Oedipus realizes that he was Laius's murderer and that he's been married to his mother... which they even had children talk about incest. The overall connection between both stories is the tragic end, Oedipus slpwt with his mother, and when she found out she slept with her son she took her life, similar to when juliet took her life after she saw that romeo had poisend himself.

Let me break things down for you a little better. Star crossed lovers are two people - who seem to not end up with each other. It's as if fate already has it's own plan together - and they do not belong together whatsoever. Now Romeo and Juliet had both tragically ended their own lives at the end of the play - leaving them unable to finally spend their lives together. While Oedipus the King had also ended his life because he found out that he'd in the end led to Laius' (his father's) demise - and that he was marrying his own mother - and also had children with her. They were starcrossed because they were related - and their story ended in tragedy because it wasn't meant to be.
If you have any questions, or need anything let me know! I hope all is well, and you pass! (: Good luck, rockstar!