Generally you can use tex symbols in Veusz. Therefore, you can write \Omega_{DE} and \Omega_{k} for your request. See details here (Sec. 2.4 Text).
Veusz understands a limited set of LaTeX-like formatting for text. There are some differences (for example, "10^23" puts the 2 and 3 into superscript), but it is fairly similar. You should also leave out the dollar signs. Veusz supports superscripts ("^"), subscripts ("_"), brackets for grouping attributes are "{" and "}".
Other LaTeX commands are supported. "\" breaks a line. This can be used for simple tables. For example "{a\b} {c\d}" shows "a c" over "b d". The command "\frac{a}{b}" shows a vertical fraction a/b.
Also supported are commands to change font. The command "\font{name}{text}" changes the font text is written in to name. This may be useful if a symbol is missing from the current font, e.g. "\font{symbol}{g}" should produce a gamma. You can increase, decrease, or set the size of the font with "\size{+2}{text}", "\size{-2}{text}", or "\size{20}{text}". Numbers are in points.
Various font attributes can be changed: for example, "\italic{some italic text}" (or use "\textit" or "\emph"), "\bold{some bold text}" (or use "\textbf") and "\underline{some underlined text}".
Example text could include "Area / \pi (10^{-23} cm^{-2})", or "\pi\bold{g}".
Veusz plots these symbols with Qt's unicode support. You can also include special characters directly, by copying and pasting from a character map application. If your current font does not contain these symbols then you may get a box character.
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In addition to the answer OmG posted, you can also directly enter the character (via a character map application or copy and paste), as Veusz supports unicode characters.
Generally you can use tex symbols in Veusz. Therefore, you can write
\Omega_{DE}
and\Omega_{k}
for your request. See details here (Sec. 2.4 Text).