Foreign students did well in the O-level. This is not surprising. They have been in our schools for a long time and generally they are very serious in their studies.
Around the mid 90s, there was already quite a strong presence of foreign students in a few secondary schools I knew of. Many of these students were from China and Indonesia, some from Malaysia, Thailand and HK and a few from Taiwan and Cambodia.
Many entered the school direct into sec 1 or 2 (not through PSLE) but they had to take an entrance exams conducted by the school. It usually consisted of an English paper, a maths paper and maybe a science paper. They paid full-rate school fees and I think they had to donate a sum to the school if admitted.
The China students were generally weak in English but good in Mathematics. But usually many managed to get a comfortable credit in English after 3 or 4 years with sheer determination and perseverance. But there were also a few who had to be retained or transferred to another stream because they failed English.
Naturally Singapore parents are worried that foreign students would compete with their children not only in schools but also compete for places in institutions of higher learning, for scholarships and later for jobs.