We can add the digits 1 through 9, omitting zero which is difficult to distinguish from the letter O, to the list of capital (uppercase) letters, yielding 45. Use lining figures (uppercase numerals) not text figures.
We assume that font and resolution sufficient to differentiate OQ can also differentiate 1I 2Z 5S 6G 8BΘ OΘ etc.
There are also lowercase letters with ascenders but I don't feel they work so well in this collection. In some fonts, J and Q (and maybe more) have descenders.
For base 45, it might be better to use zero and omit capital letter O so that the symbols can use a natural order: numbers, Latin letters, Greek letters. Omega is last, following tradition.
Perhaps useful for a mysterious-looking wall of symbols like The Matrix. Here is some random text: W