Can a student with a bachelor's in engineering with very little chemistry take some courses in chemistry to get into a chemistry masters? How do US masters view courses taken from Open University, particularly in chemistry?
Some of this may be superfluous but the US system is very different from anything in UK or EU. In the first place there is no national system at all. Even within a single state, policies can differ widely, though the official state university system (so called public colleges, but not the same idea as UK public colleges) will usually share a set of policies within that state. But many schools, including some very highly rated ones (Stanford, Harvard, Yale, etc. i.e "Private colleges") aren't part of any system. Some "flagship" schools within a state may also have more rigorous admissions than the rest of the state system (UCBerkeley, UMichigan, etc.)
While Open University in general is highly regarded, each college (or system of colleges) will have its own admissions policies. These can even differ by discipline, so Chemistry might be different from Mathematics. Some admissions systems are more personalized, depending on interviews, and some have various requirements on grade averages, required courses, and other things.
The only real way to know if X is acceptable at a US university is to ask that institution or to apply formally. If your situation is non standard you might get asked for additional information. If an educational program is very popular and also highly regarded the unusual cases might be easier to simply reject as there are enough very highly qualified applicants who are easy to judge.
In general, however, a Masters degree application won't face extremely rigorous requirements. This may be different from European experience. It would be a different story for doctoral study, of course.
With all that said, I would guess that if you meet a particular school's requirements for prerequisite courses, roughly what an undergraduate would be expected to have in that discipline, and your grades were good, and you represent yourself well in the application, then I doubt that having studied at Open U would be an issue - most places. But the only way to know is to ask the institution in question and expect to get different answers from different institutions.
BTW, students in the US think of this "system" as perfectly normal.
I am a Chemistry graduate and I have been working in many engineering departments of my university for years. Especially in Turkey, engineering salaries are way higher than the ones for graduates of chemistry, physics, biology etc. if they can ever find a job. This mere fact makes everyone thinks that those pure science disciplines are just trash and an engineer, of course, can compensate the difference in a couple of months and get a degree from Chemistry, even an M.Sc.
Even normal 4 year curriculum of chemistry is not sufficient to comprehend the functioning of chemistry. You may get a degree still, as system allows it, but you will not make a contribution to neither pure nor applied science. Even though you may think that I am not answering your questions, you should still understand that unlike engineering courses, you can't just go listen the lecture, calculate the answers of the questions and submit the assignments in time than, boom, you are an expert in chemistry. I have taken both undergraduate and graduate courses from Civil Engineering, Geological Engineering, Environmental Engineering, Biology, Chemistry and even some others.
More properly, there are branches of Chemistry (even though not in real world) like analytical, bio, organic, inorganic, physical, quantum/computational and even industrial. Which one of them you will compensate with a couple of courses? You can never truly understand one branch without knowing sufficiently of others.
Don't take it personal, I, in general, don't suggest others to leisurely waltz into hard science and try to get a degree.