The purpose of US High School Diploma is equivalent to our school-leaving certificate. It is only used to confirm that you've accomplished high school courses at a stated high school.
It can only be used for admission to very low-tier colleges, as top-tier colleges only considered it merely as confirmation certificate to prove to the admission committee that you've really taken high school courses at particular high school.

The existance of score reports of the widely-acclaimed Advanced Placement (AP) tests, SAT tests, ACT tests and so had made the purpose of the US High School Diploma as the indicator of rigor and aptitude for admission obsolete, reducing the evolved purpose back to merely a school-leaving certificate.
Btw, OP, why did you asked? Are you currently taking it or so?

Of course, I understand. What I'm trying to convey is that the High School Diploma is not sufficient for top-tier colleges, undoubtedly if without test scores or the IB if offered in school.
About your reply, I need to ask for a clarification.. You did your O-Level before or after High School Diploma?

If by stages, it would consider equivalent to STPM, as it is conferred at the end of Grade 12, equivalent to lower six.
However, I guess that, without your transcript to prove higher rigor with either AP courses, honors classes or dual enrollments in community colleges, it won't be enough with just ordinary courses, because the ordinary courses are not equivalent to our Commonwealth curriculum. They're easier.
Plus the fact that we end our high school at 13 grades in total, and our curriculum at the Grade 10 already at par with their 11th and so on, the curriculum of US High School without extra courses lost to ours a lot.
Didn't you take any standardized test?

Or just proceed to a low-tier college in the United States that accepts the US High School Diploma, and opts out for test scores.
These schools require no test scores and use US High School and the high school transcript, which means a thorough review of your high school classes rigor.
Unless of course, if OP wants to proceed further education in Malaysia.