Hi, and welcome to TalkingScot
Folk generally start with a question such as "My g-g-grandmother was......."
The answer to your question could, (and has done,) fill a book.
The most complete answer can be found in the explanatory pages of the Scotlandspeople site itself.
You will find details about parishes, availibility or otherwise of Old Parochial records for a particular parish, changes/amalgamations in parish boundaries and a host of other information.
What it cannot tell you is how useful the records of a particular parish may be to your unique search. Entries vary enormously in both quantity and quality. Some give details unavailable anywhere else. Others give the bare facts that 'a male/female child was born to J.............on a given date.
Same parish, different minister a few years earlier or later could give names, occupations, sometimes spouses father's name, where they lived.
The outcome of a search is a gamble. One of my recent OPR searches turned up triplets all on the same page of the Register and we hadn't even known about them.
Searches can be done by name and approximate Birth or Marriage dates. In earlier records sometimes only the father's name is recorded which makes corroboration more difficult. If you have the names of parents it is possible to search for siblings by keying in the surname; gender - both; parents names; and a broad span of years covering their productive married life.
The Wild cards '?' and '*' can be used to cover possible spelling variants but instruction for this is also on scotlandspeople.
If you search for a Marriage using only one name you may turn up quite a few within a restricted time span particularly if it is a common name.
If you know the parish you may still find that there are several of that name from the same time period.
Because the OPR contains few Death entries the best way to establish them is to chase Monumental Inscriptions and Lair records in cemeteries.
I now have a couple of families going back to the mid 1600's thanks to the OPR but if you have not prepared your search parameters carefully before you start it can prove expensive with lots of wrong results.
Nobody said this hobby/obsession was a cheap one.
I hope this has answered your main questions
Russell
Working on: Oman, Brock, Miller/Millar, in Caithness.
Roan/Rowan, Hastings, Sharp, Lapraik in Ayr & Kirkcudbrightshire.
Johnston, Reside, Lyle all over the place !
McGilvray(spelt 26 different ways)
Watson, Morton, Anderson, Tawse, in Kilrenny