Ã/ã and Õ/õ are special characters in Portuguese. They spell nasal vowels, as in the words
são = Spanish
san and
não = Spanish
no.
The standard English (US) and English (UK) keyboard layouts do not provide a way to type these letters, just as they do not allow a way to type the special characters for Spanish. I have configured my computer so that I can easily switch between English (US) and Spanish (US) as the input language, and the keyboard layout for Spanish (US) provides a way to type the Portuguese letters.
If you haven't seen it already, you can find some information about entering special characters
here.