You want the “Circled Dot Operator” unicode character (⊙), right?
In a [blank if you like] Writer document, type 2299 then press Alt+X. Alt+X turns the hexadecimal number directly to the left of the cursor into the corresponding unicode character. You then copy it into the formula.
I found this character by searching for “circled dot” in the “gucharmap” utility in Linux. Another match was 2A00 (⨀), which is described as “N-ary Circled Dot Operator”.
edit: I noticed after posting that @regina beat me to it as I was researching and typing, but I’ll leave this here in case it’s helpful, since I mention some different aspects.