ΔE 15.0, loose: different enough that you should pick on purpose. Blue Mountain is noticeably lighter, warmer than Deep Ocean Blue.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Blue Mountain is noticeably lighter, warmer than Deep Ocean Blue.
The same difference applies the other way round: Deep Ocean Blue is noticeably darker, cooler.
ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.
See Deep Ocean Blue on the colour wheel See Blue Mountain on the colour wheel
All matches: Deep Ocean Blue equivalents · Blue Mountain equivalents