OMSSA Parser

OMSSA Parser Publications:

SearchGUI and PeptideShaker:

  • To run OMSSA searches we recommend the use of SearchGUI.
  • To visualize and analyze OMSSA results we recommend the use of PeptideShaker.

download v1.9.0 - All platforms ReleaseNotes


omssa parser is a java based parser for omssa (open mass spectrometry search algorithm.html) omx files.

Initially created by Steffen Huber for Prof. Dr. Albert Sickmann under the guidance of Dr. Lennart Martens, and further developed by Harald Barsnes, including the addition of a simple, lightweight and platform independent OMSSA Viewer.

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Using OMSSA Parser

There are basically two ways of using OMSSA Parser:

  • Using the OMSSA Viewer to visualize the contents of an omx file, including the possibility of exporting parts of the dataset to other file formats.
  • Using the OMSSA Parser Jar File as Java library in your own programs to extract the parts of the dataset you need.

OMSSA Viewer

To visualize and analyze OMSSA results we now recommend the use of PeptideShaker.

OMSSA Viewer was mainly created for showing the usefulness of OMSSA Parser and as an example of how to use the parser. It displays the spectra file details (spectrum number, filename, charge and m/z-value), the identification details for a selected spectra (peptide sequence, E-value, protein accession number, etc.), and the selected spectrum including ion coverage. All in the same frame. OMSSA Viewer also includes the possibility of exporting the different components of an omx file as tab delimited text files, and all the spectra as dta files. A screenshot of OMSSA Viewer can be found here.

Using OMSSA Viewer is straight-forward. Simply download the latest version of the OMSSA Parser zip file, unzip the file and double click on the jar file. (If nothing happens, download Java 1.5 (or newer) and try again. If this does not fix your problem, see the Troubleshooting section below.) (See also screenshot.)

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OMSSA Parser Jar File

Using OMSSA Parser directly requires that you create your own Java project that uses the OMSSA Parser jar file as a library.

The library consists of three packages: (i) de.proteinms.omxparser, (ii) de.proteinms.omxparser.util and (iii) The de.proteinms.omxparser package only contains the OmssaOmxFile class which manages the parsing of the omx file, and provides several methods to easily retrieve the parsed information.

To parse an omx file into an OmssaOmxFile object use the class’s constructor with the following input parameters: (i) the omx file, (ii) the OMSSA mods.xml file, and (iii) the OMSSA usermods.xml file. (The latter two are the files where OMSSA stores the properties of the amino acid modifications.) Only the first file is mandatory, but if the modification files are not provided no additional information about the modifications, besides a modification number, can be extracted. The creation of the OmssaOmxFile object creates an OmxParser object that does the actual parsing of the omx file. Creating an OmssaOmxFile also takes care of processing the results and creates several data structures that make it easier to extract commonly used information from the omx file. Among these is the method getParserResult(), that returns all data from the original omx file as an MSSearch object, and the method getSpectrumToPeptideMap(), that returns a HashMap where every spectrum is allocated to its corresponding peptides.

The OmxParser class is located in the de.proteinms.omxparser.util package along with the other classes needed for the actual parsing. The structure of the OMSSA Parser code is closely related to the omx file structure. When using the parser it is therefore recommended to have a copy of OMSSA.mod.xsd available as a reference.

The example omx file contains excerpts of an omx file. As an example of how to use OMSSA Parser we describe the code for extracting the m/z-values of all the spectra in a file (see example omx file - Spectra):

OmssaOmxFile omxFile = new OmssaOmxFile(C:\\OMSSA_Files\\BSA.omx);

HashMap<MSSpectrum, MSHitSet> results = omxFile.getSpectrumToHitSetMap();
Iterator<MSSpectrum> iterator = results.keySet().iterator();

ArrayList<List<Integer>> allMzValues = new ArrayList();

while (iterator.hasNext()) {
    MSSpectrum tempSpectrum =;

The OMSSA Parser code uses public fields to move from a tag to its subtags, e.g., MSSpectrum_mz.MSSpectrum_mz_E moves from the MSSpectrum_mz tag to its subtag MSSpectrum_mz_E, and uses linked list whenever a subtag can occur more than once, e.g., MSSpectrum_mz_E is a linked list of integer values containing all the m/z-values for a given spectrum.

The last package in OMSSA Parser is, which contains a simple, lightweight viewer for OMSSA omx files called the OMSSA Viewer.

In addition to the packages detailed above, OMSSA Parser also requires the following four libraries: xpp3-1.1.4c.jar (for the XML parsing), utilities-2.9.jar (for the spectrum graph), looks-2.2.0.jar (for the graphical user interface) and log4j-1.2.15.jar (for logging functionality). All of these are available in Maven compliant repositories. They are also included in the OMSSA Parser zip file.

Modification Details

In the omx file the only information present about a given amino acid modification is its location in the peptide and a modification reference number. To extract additional information about a modification, e.g., the type or the modification mass, the two XML files mods.xml and usermods.xml are needed. These are normally located in the OMSSA installation folder, and if provided allows you to map a given modification reference number to an OmssaModification object containing the details about the modification. Use the method getModifications() in the OmssaOmxFile class to extract this information after parsing an omx file.

OMSSA Enumerations

The OMSSA OMX file uses several enumerations. When parsing a tag referring to an enumeration an integer ID is return. To map this ID to the corresponding text element, use the OmssaEnumerators class. This class has maps for all enumerations, e.g., MSEnzymes, MSIonType etc. By providing the integer ID the corresponding text element wil be returned.

Scaled Values

Note that all the m/z and abundance values in the omx file are stored as integers, and needs to be scaled to get the real values. Each spectrum has its own abundance scale, MSSpectrum_iscale, while the m/z-scales are given by MSSearchSettings_scale and MSResponse_scale.

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Result Analysis

To visualize and analyze OMSSA results we recommend the use of PeptideShaker. PeptideShaker is a search engine independent platform for visualization of peptide and protein identification results from multiple search engines.

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Maven Dependency

OMSSA Parser is available for use in Maven projects:


    <!-- Compomics Genesis Maven 2 repository -->
        <name>Genesis maven2 repository</name>

    <!-- old EBI repository -->
        <name>The EBI internal repository</name>

    <!-- EBI repository -->
        <name>EBI Nexus Repository</name>


Update the version number to latest released version.

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Memory Settings

If you are parsing a large omx file (say bigger than 100MB), you’ll need to allow for more memory space for the Java Virtual Machine (JVM). You can do this by appending the following start-up arguments to the java command:

     java -Xmx999m ...

where you can substitute the ‘999’ by the maximum amount of megabytes you want the JVM to access.

When using OMSSA Viewer the memory boundaries can be set it the JavaOptions.txt file in the ../Properties folder using the same syntax as above.

Note that most 32 bit platforms will only allow memory allocations up to 1500 megabytes.


Note that the OMSSA OMX file may contain references to the Bioseq module/schema FROM NCBI-Sequence. These elements are not currently parsed by OMSSA Parser. However, files containing such elements are of course parsed without errors.

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(Click on figure to see the full size version)

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Example OMX File


<MSSearch ...>

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<MSSearch ...>
                  <MSPepHit_defline>Serum albumin precursor (Allergen Bos d 6) (BSA)</MSPepHit_defline>
            <MSHitSet_ids_E> LCQ10486.10.10.1.dta</MSHitSet_ids_E>

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