It really depends on the keyboard layout (e.g. Croatian) you have selected for your system.
In my experience, US/British layouts are almost identical between Windows and OS X. The safe characters are usually those you can type on both keyboard by using no or only the
Shift modifier key. Incidentally, they are always printed on the keyboard, so you can quickly figure out which are the same on both your systems.
If you don't have a physical Mac keyboard, you can always open the keyboard viewer via System Preferences » Language & Text » Input Sources » Show Input menu in menu bar and the menu's Show Keyboard Viewer item.
Mac OS X simply does not support Alt codes as part of regular keyboard layouts. There is a Unicode Hex Input keyboard layout, but it will break regular use of the
Option key, e.g. to navigate words (
Option-Shift-Right, etc., as well as the regular
Option keyboard shortcuts).
On Windows, many keyboard shortcuts trigger with
Altplus some other key (including menu bar items), so that's probably not a useful solution.