diff --git a/sentry_sdk/opentelemetry/span_processor.py b/sentry_sdk/opentelemetry/span_processor.py
index abfb712a89..b5279bccb0 100644
--- a/sentry_sdk/opentelemetry/span_processor.py
+++ b/sentry_sdk/opentelemetry/span_processor.py
@@ -289,8 +289,12 @@ def _span_to_json(self, span):
         if parent_span_id:
             span_json["parent_span_id"] = parent_span_id
 
-        if span.attributes:
-            span_json["data"] = dict(span.attributes)
+        attributes = getattr(span, "attributes", {}) or {}
+        if attributes:
+            span_json["data"] = {}
+            for key, value in attributes.items():
+                if not key.startswith("_"):
+                    span_json["data"][key] = value
 
         return span_json
 
diff --git a/sentry_sdk/tracing.py b/sentry_sdk/tracing.py
index 388cf38cef..92ac4d7671 100644
--- a/sentry_sdk/tracing.py
+++ b/sentry_sdk/tracing.py
@@ -588,10 +588,10 @@ def set_context(self, key, value):
 
     def set_flag(self, flag, value):
         # type: (str, bool) -> None
-        flag_count = self.get_attribute("flag.count") or 0
+        flag_count = self.get_attribute("_flag.count") or 0
         if flag_count < _FLAGS_CAPACITY:
             self.set_attribute(f"flag.evaluation.{flag}", value)
-            self.set_attribute("flag.count", flag_count + 1)
+            self.set_attribute("_flag.count", flag_count + 1)
 
 
 # TODO-neel-potel add deprecation
diff --git a/tests/integrations/threading/test_threading.py b/tests/integrations/threading/test_threading.py
index 4ab742ff1f..8a5dfef62b 100644
--- a/tests/integrations/threading/test_threading.py
+++ b/tests/integrations/threading/test_threading.py
@@ -9,7 +9,6 @@
 import sentry_sdk
 from sentry_sdk import capture_message
 from sentry_sdk.integrations.threading import ThreadingIntegration
-from sentry_sdk.tracing import _OTEL_VERSION
 
 original_start = Thread.start
 original_run = Thread.run
@@ -106,10 +105,7 @@ def double(number):
         assert len(event["spans"]) == 0
 
 
-@pytest.mark.skipif(
-    sys.version[:3] == "3.8" and (1, 12) <= _OTEL_VERSION < (1, 16),
-    reason="Fails in CI on 3.8 and specific OTel versions",
-)
+@pytest.mark.skipif(sys.version[:3] == "3.8", reason="Fails in CI on 3.8")
 def test_circular_references(sentry_init, request):
     sentry_init(default_integrations=False, integrations=[ThreadingIntegration()])
 
diff --git a/tests/opentelemetry/test_span_processor.py b/tests/opentelemetry/test_span_processor.py
new file mode 100644
index 0000000000..7d6283d4ea
--- /dev/null
+++ b/tests/opentelemetry/test_span_processor.py
@@ -0,0 +1,19 @@
+import sentry_sdk
+
+
+def test_span_processor_omits_underscore_attributes(sentry_init, capture_events):
+    sentry_init(traces_sample_rate=1.0)
+
+    events = capture_events()
+
+    with sentry_sdk.start_span():
+        with sentry_sdk.start_span() as span:
+            span.set_attribute("_internal", 47)
+            span.set_attribute("noninternal", 23)
+
+    assert span._otel_span.attributes["_internal"] == 47
+    assert span._otel_span.attributes["noninternal"] == 23
+
+    outgoing_span = events[0]["spans"][0]
+    assert "_internal" not in outgoing_span["data"]
+    assert "noninternal" in outgoing_span["data"]
diff --git a/tests/test_feature_flags.py b/tests/test_feature_flags.py
index 1b0ed13d49..5c2f1cd352 100644
--- a/tests/test_feature_flags.py
+++ b/tests/test_feature_flags.py
@@ -259,3 +259,19 @@ def test_flag_limit(sentry_init, capture_events):
         }
     )
     assert "flag.evaluation.10" not in event["spans"][0]["data"]
+
+
+def test_flag_counter_not_sent(sentry_init, capture_events):
+    sentry_init(traces_sample_rate=1.0)
+
+    events = capture_events()
+
+    with start_transaction(name="hi"):
+        with start_span(op="foo", name="bar"):
+            add_feature_flag("0", True)
+            add_feature_flag("1", True)
+            add_feature_flag("2", True)
+            add_feature_flag("3", True)
+
+    (event,) = events
+    assert "_flag.count" not in event["spans"][0]["data"]