These days you can just copy the URL from your browser address bar which should look something like this (with your example coordinates) :
https://www.openstreetmap.org/#map=16/30.486/-90.195
(Note: this dynamically updated '#' character URL structure was introduced some time after the original question was asked)
OpenStreetMap offers quite a lot of other options for linking to the maps, some of which are available by clicking the "Share" button to access a right-hand panel there. These are also described on the documentation about browsing the map.
e.g.
https://www.openstreetmap.org/?mlat=30.486&mlon=-90.195#map=16/30.486/-90.195 ...puts a marker at those coordinates
http://osm.org/go/Tu_z5cFR ...as a 'short link' (redirects)
http://osm.org/go/Tu_z5cFR?m ...a 'short link' with marker
You can actually slap in a coordinate pair "30.486,-90.195" with a comma into the search box on the OpenStreetMap homepage, and that gives you a link to click, which dynamically zooms you to that spot, although you can also right click to open the link in a new window at a URL formatted as above.
If you want a frame embeddable map, use the Share panel 'HTML' option documented here which gives you HTML like this:
<iframe width="425" height="350" frameborder="0" scrolling="no" marginheight="0" marginwidth="0" src="http://www.openstreetmap.org/export/embed.html?bbox=-90.20224,30.48102,-90.18775,30.49097&layer=mapnik" style="border: 1px solid black"></iframe><br /><small><a href="http://www.openstreetmap.org/?lat=30.485995000000003&lon=-90.194995&zoom=15&layers=M">View Larger Map</a></small>
Contained with that is a URL like this...
http://www.openstreetmap.org/export/embed.html?bbox=-90.20224,30.48102,-90.18775,30.49097&layer=mapnik
All of these URL formats could be generated by Excel using a formula to go from the lat/lon values in two other cells.