Example of my .Rprofile

.Rprofile

# If loading in an environment that doesn't respect my RPATH environment
# variable, set it here
sourceDir <- file.path(path.expand("~"), "R", "source")
if (Sys.getenv("RPATH") == "") {
	if (!file.exists(sourceDir)) dir.create(sourceDir)
	Sys.setenv(RPATH=sourceDir)
}

# Load commonly used functions and packages

# enable auto-completion of package names
utils::rc.settings(ipck=TRUE)
# Enable more than 100 packages loaded
 if (getRversion() >= "3.4.0") Sys.setenv("R_MAX_NUM_DLLS"=300)

# Load custom functions from util.R file (see in https://gist.github.com/IdoBar/7f63547158ecdbacf31b54a58af0d1cc)
# if (interactive()){
# if (require(devtools) & curl::has_internet()) devtools::source_gist("7f63547158ecdbacf31b54a58af0d1cc", filename = "util.R")
# if (file.exists(search.path(utilsFileName))) source(search.path(utilsFileName))
# }
# Setup favourite CRAN mirror and a site install library.
# Choose CRAN mirror (1 will use https://cloud.r-project.org/ to Automatically redirect to servers worldwide through ) 
utils::chooseCRANmirror(ind = 1)
# fix "Warning: unable to access index for repository https://www.stats.ox.ac.uk/pub/RWin/src/contrib"
# https://stackoverflow.com/questions/45108484/warning-unable-to-access-index-for-repository-https-www-stats-ox-ac-uk-pub-rw#45112048
options(repos = getOption("repos")["CRAN"])
# options(repos=structure(c(CRAN="https://cloud.r-project.org/")))
# myLib <- "C:/Users/Public/R/MRO-3.3.1/library"
# .libPaths(c(myLib, .libPaths()))

# Function to run when R is started
.First <- function() cat(sprintf("\n\nDefault location to install new packages: '%s' \nCRAN mirror is set to: '%s'. \n\n	Welcome to R!\n\n", 
.libPaths()[1], getOption("repos")["CRAN"]))

Remove CRANExtra repository which causes warning (and leads to errors in the installation of some packages).
See this SO thread.