Looking at Interface's glyph map, I see that letter-O (O) is slightly wider than number-zero (0). Capital-eye (I) is indistinguishable from small-ell (l), though number-one (1) is distinguishable from both.
What other glyph ambiguities do you look out for on new fonts?
This is by design. A wider "O" would indeed make separating zero from "O" easier (and an earlier iteration of Interface had a wider "O") but it adds some uneven pace to how longer words and sentences read.
The hope is to add a stylistic set for character disambiguation to the font, which when enabled would enable graphic features on glyphs like upper-case "i" and lower-case "l", zero and "O", and so on. Tracked here: https://github.com/rsms/interface/issues/1
Though not abiguous in sharing the exact same shapes, many characters can share similar structures that (arguably; citation needed) can cause visual confusion. For example;
3 and 8 in a typeface like Helvetica are very similar. Opening the aperture of the 3 or using a form with a corner in the upper right can be observed in some UI typefaces.
The "single story a" can be an issue in typefaces that ise it, adding to the plethora of round forms (eopdqbc etc.). Breaking up the monotony of those shapes is also frequently advised.
"Tail-less" t and r in sans type can be problematic at small sizes.
What other glyph ambiguities do you look out for on new fonts?