Cool! There's an easy tip for "o": If it's in the dictionary as о́ then it's the stressed syllable and you say the "o", otherwise it's "a". Example word: о́вощи, the first "o" is stressed. You should include the accent mark on any flashcards you make, since it can literally change the word you are saying (сто́ит, стои́т).
In my experience, the unstressed "o" as "a" is only mostly true and varies by region. This is especially true for unstressed "o" preceding the stressed vowel.
For example, "молоко" is pronounced differently in different parts of the country. For me, the first "o" sounds more like a "ə" (think bOOk) around Moscow, and more like "a" further east.
https://en.wiktionary.org/wiki/овощи https://en.wiktionary.org/wiki/стоит