Which version of LO you are using? I have done some testing under Ubuntu 10.04 x86_64 + LO v3.5.7.2, Ubuntu 10.04 x86_64 + LO v4.0.1.2, and Win 7 HP + LO v4.0.1.2. There does appear to be an initial artifact issue under Win7 and a more general problem with minus sign treatment. An example of the particular formula code you are using would also be helpful. Test formula code used in the examples below:
{the brace is above}
overbrace a
{the brace is below}
underbrace {f} newline
sqrt { { { df } ( x ) } over { dx } } = sqrt { ln ( x )  tan^1 ( x^2 ) } newline
x  y newline
x − y newline
x {""} y newline
x {"−"} y newline
References are made to the LO v3.5 Math Guide. There are several potential issues, so I will address each separately:

Windows 8: Refer my answer here. You may have a platformspecific issue.

Formula spacing: Refer my answer here. There are some known issues with spacing. Again, this may relate to your situation.

Overbrace / underbrace positioning: I have used the (basic) example from p. 47 of the Guide. It displays as expected under Ubuntu. An ODT containing formulas created under Ubuntu, saved, and subsequently opened under Win7 has some rather bad visual artifacts e.g., as you indicate an over/underbrace does not align correctly with the related text. Editing (entering) the formula rectifies the display artifact, but this then requires the file be saved. I can find no over/underbrace related bug other than FDO#55086, which deals with overlapping over/underbraces, which I hope you are not dealing with.

Square root sign positioning: I have used a modified version of the example from p. 5 of the Guide. As for previous point. An ODT created on Ubuntu displays the square root crossbar in misalignment with the initial radical sign (√). This appears to be covered by FDO#60268 and FDO#61886.

Minus sign spacing: In terms of platform display consistency, this sign does not appear to be an issue (and I acknowledge that this was not raised as an issue). The particular character displayed though is more problematic. Currently the LO Math engine appears to convert the hyphenminus keyboard character (U+002b) in a formula to what appears to be an emdash (U+2014). Perhaps there is some precedent for this, but I would think it more appropriate to substitute the mathematical minus sign (U+2212). FDO#49547 appears to be related. The following formula codes all produce slightly different spacing:

x  y
(U+002b)

x − y
(U+2212)

x {""} y
(U+002b)

x {"−"} y
(U+2212)
The spacing issue is largely (but not entirely) influence by font (which I mention in the Formula spacing link above). This is how these formula elements display under Ubuntu. This is how these formula elements initially display when opened under Win7. And this is how these formula elements display after editing (on the left) under Win7.