diff --git a/NAMESPACE b/NAMESPACE
index 43d31537..76b6c12d 100644
--- a/NAMESPACE
+++ b/NAMESPACE
@@ -1,5 +1,6 @@
 # Generated by roxygen2: do not edit by hand
 
+export(ExpHazOS)
 export(ExpQuantOS)
 export(ExpSurvOS)
 export(ExpSurvPFS)
diff --git a/R/hazardFunctions.R b/R/hazardFunctions.R
new file mode 100644
index 00000000..8b82a4f2
--- /dev/null
+++ b/R/hazardFunctions.R
@@ -0,0 +1,21 @@
+#' OS Hazard Function from Constant Transition Hazards
+#'
+#' @param t (`numeric`)\cr study time-points.
+#' @param h01 (positive `number`)\cr transition hazard for 0 to 1 transition.
+#' @param h02 (positive `number`)\cr transition hazard for 0 to 2 transition.
+#' @param h12 (positive `number`)\cr transition hazard for 1 to 2 transition.
+#'
+#' @return This returns the value of the OS hazard function at time t.
+#' @export
+#'
+#' @examples
+#' ExpHazOS(c(1:5), 0.2, 1.1, 0.8)
+ExpHazOS <- function(t, h01, h02, h12) {
+  assert_numeric(t, lower = 0, any.missing = FALSE)
+  assert_positive_number(h01)
+  assert_positive_number(h02)
+  assert_positive_number(h12)
+
+  h012 <- h12 - h01 - h02
+  ((h12 - h02) * (h01 + h02) - h01 * h12 * exp(-h012 * t)) / ((h12 - h02) - h01 * exp(-h012 * t))
+}
diff --git a/_pkgdown.yaml b/_pkgdown.yaml
index 9ffeb1c0..1d5659e5 100644
--- a/_pkgdown.yaml
+++ b/_pkgdown.yaml
@@ -63,3 +63,6 @@ reference:
       - PWCsurvPFS
       - PwcOSInt
       - PWCsurvOS
+  - title: Hazard functions
+    contents:
+      - ExpHazOS
diff --git a/man/ExpHazOS.Rd b/man/ExpHazOS.Rd
new file mode 100644
index 00000000..5b6b99ed
--- /dev/null
+++ b/man/ExpHazOS.Rd
@@ -0,0 +1,26 @@
+% Generated by roxygen2: do not edit by hand
+% Please edit documentation in R/hazardFunctions.R
+\name{ExpHazOS}
+\alias{ExpHazOS}
+\title{OS Hazard Function from Constant Transition Hazards}
+\usage{
+ExpHazOS(t, h01, h02, h12)
+}
+\arguments{
+\item{t}{(\code{numeric})\cr study time-points.}
+
+\item{h01}{(positive \code{number})\cr transition hazard for 0 to 1 transition.}
+
+\item{h02}{(positive \code{number})\cr transition hazard for 0 to 2 transition.}
+
+\item{h12}{(positive \code{number})\cr transition hazard for 1 to 2 transition.}
+}
+\value{
+This returns the value of the OS hazard function at time t.
+}
+\description{
+OS Hazard Function from Constant Transition Hazards
+}
+\examples{
+ExpHazOS(c(1:5), 0.2, 1.1, 0.8)
+}
diff --git a/tests/testthat/test-hazardFunctions.R b/tests/testthat/test-hazardFunctions.R
new file mode 100644
index 00000000..3115533c
--- /dev/null
+++ b/tests/testthat/test-hazardFunctions.R
@@ -0,0 +1,15 @@
+# ExpHazOS ----
+
+test_that("ExpHazOS works as expected", {
+  actual <- ExpHazOS(1, 0.2, 1.1, 0.8)
+  expect_equal(actual, 1.038192, tolerance = 1e-3)
+
+  actual2 <- ExpHazOS(2, 0.4, 1.4, 0.1)
+  expect_equal(actual2, 0.266345, tolerance = 1e-3)
+})
+
+test_that("ExpHazOS works also with vector of times t", {
+  result <- ExpHazOS(t = c(1:3), 0.2, 1.1, 0.8)
+  expected <- c(1.0381919, 0.9777975, 0.9253826)
+  expect_equal(result, expected, tolerance = 1e-3)
+})