diff --git a/arxiv.py b/arxiv.py
index f9dc958..ab5b563 100644
--- a/arxiv.py
+++ b/arxiv.py
@@ -1,3 +1,6 @@
+"""
+This file contains all the arXiv-specific functions.
+"""
 import bbl
 import io
 import requests
diff --git a/bbl.py b/bbl.py
index 97ace12..63d6e95 100644
--- a/bbl.py
+++ b/bbl.py
@@ -1,3 +1,6 @@
+"""
+This files contains all the functions to deal with bbl files.
+"""
 import doi
 import math
 import os
diff --git a/doi.py b/doi.py
index 76c69a6..46a66ba 100644
--- a/doi.py
+++ b/doi.py
@@ -1,3 +1,8 @@
+"""
+This file contains all the DOI-related functions.
+"""
+import requests
+
 import regex
 import tools
 
@@ -75,3 +80,27 @@ def match_doi_or_arxiv(text, only=["DOI", "arXiv"]):
         if extractID:
             return ("arXiv", extractID.group(1))
     return None
+
+
+def get_oa_version(doi):
+    """
+    Get an OA version for a given DOI.
+
+    Params:
+        - doi is a DOI or a dx.doi.org link.
+
+    Returns the URL of the OA version of the given DOI, or None.
+    """
+    # If DOI is a link, truncate it
+    if "dx.doi.org" in doi:
+        doi = doi[doi.find("dx.doi.org") + 11:]
+    r = requests.get("http://beta.dissem.in/api/%s" % (doi,))
+    oa_url = None
+    if r.status_code == requests.codes.ok:
+        result = r.json()
+        if("status" in result and
+           "paper" in result and
+           result["status"] == "ok" and
+           "pdf_url" in result["paper"]):
+            oa_url = result["paper"]["pdf_url"]
+    return oa_url
diff --git a/main.py b/main.py
index f2fd229..dc5721d 100755
--- a/main.py
+++ b/main.py
@@ -7,14 +7,6 @@ import arxiv
 import bbl
 
 
-def oa_from_doi(doi):
-    """
-    Get an OA version for a given DOI.
-    """
-    # http://beta.dissem.in/api/10.1088/1367-2630/17/9/093036
-    pass
-
-
 if __name__ == "__main__":
     import pprint
     if len(sys.argv) < 2:
diff --git a/regex.py b/regex.py
index 940f2de..584d8ef 100644
--- a/regex.py
+++ b/regex.py
@@ -1,3 +1,6 @@
+"""
+This file contains all the regex used across the app.
+"""
 import re
 
 urls = re.compile(r"http[s]?://(?:[a-zA-Z]|[0-9]|[$-_@.&+]|[!*\(\),]|(?:%[0-9a-fA-F][0-9a-fA-F]))+")
diff --git a/tools.py b/tools.py
index c793256..e0728fd 100644
--- a/tools.py
+++ b/tools.py
@@ -1,3 +1,8 @@
+"""
+This file contains various utility functions.
+"""
+
+
 def replaceAll(text, dic):
     """Replace all the dic keys by the associated item in text"""
     for i, j in dic.items():