The precise answer to this question depends on what one is attempting to do exactly. In general, put all of your other event function calls prior to the overlib call, with each one being separated with a semi-colon (;) as shown here:

onMouseOver="function1_call(...); function2_call(...); return overlib('Some text goes here', other parameters);"

If you are not going to display anything in the status bar, you can dispense with the return value from overlib and just call it with pre-pending with return. If this is the case, then the functions can be called in any order; it shouldn't matter at all. If it does matter, then one of your other function calls depends on some variable that requires setting and it is up to you to determine what that is and how it should be set. This can be done in line, if there isn't a lot of variables to be set; otherwise, put that code in a separate function call. For example,

onMouseOver="function1_call(...); var param1=100; function2_call(...); return overlib('some text goes here', other parameters);"

The main point to take away from here is that if you include return as part of the call to overlib, then you must make sure that it is the last function called, otherwise no functions after that point will be called.